Joanna Lumley has gone from top model in the 60s to TV stardom across four decades. Her regal charm has her cast now as a real queen. But beneath her poised surface there's a fierce personality ready to snap against injustice

Everything with Joanna Lumley is legendary. Or phenomenal. Or extraooordinary. Unless it's awful. Or ghastly. Or appaaaaalling. Nobody enunciates with the kind of clarity and emphasis that Lumley uses. Especially when she employs added urgency. At times, it's a bit like trying to have a conversation with the entire cast of the RSC. Or Henry V, on the eve of battle. But then, there are moments when Joanna Lumley, the heroine of the Gurkhas, and scourge of immigration ministers, is a bit like Henry V, on the eve of battle. Any minute now, I think she might suggest we may step once more unto the breach.

But then, there are so many battles out there for her to fight. The Gurkhas were close to her heart, because of her father's connection with them – Major Lumley served with the 6th Gurkha Rifles – but it's just one of her many causes: there's also Free Tibet, and the Burma Campaign, and Compassion in World Farming, and Survival International, and Mind, and Sane, and Legal Aid, and tigers, and Low Incomes Tax Reform, and around 70 others. And once roused, she's magnificent. There's the ramrod-straight back, a legacy of her time at the Lucie Clayton school of modelling, and of course, The Voice. But mostly it's her commanding presence and her unimpeachable sense of rightness. She's an actor, after all, so she's knows how to deliver a line, and at times, it feels less like we're having a cosy chitchat, and more like she's rehearsing a dramatic monologue, but there's no doubting that it's also backed up by an absolute sense of moral purpose. No matter what the subject is. Here she is on litterers: "When you see somebody walk across the road, open up a bag of sweets and then just drop the wrapper, you think: What the…? Why did you do that? Why? Why? Look, there's a bin there! Why did you do that?" And education: "Don't teach them how to put a condom on! What's that got to do with anything? Teach them how to love and respect each other!"

And, best of all, the bankers. She's raging about the bankers. "I was appalled by the way we bailed out the bankers. Who then gave themselves bonuses! I found that appalling! Appalling… Where's your fucking shame?"

The Stig once said that the Top Gear celebrity who shocked him most was Joanna Lumley. She was so polite and well-heeled, he said, but cursed like a navvy. It was unnerving, he said, "like hearing the Queen swear". In fact, Lumley swears fluently and liberally, with an intensity that, at times, makes me feel a small twinge of sympathy for Phil Woolas, the Labour immigration minister she nobbled at BBC studios in 2009 and proceeded to dictate government policy to. There are a few times during the interview when I catch a bit of Lumley shrift ("I can see by your face that you don't believe a word I'm saying!"), and after the warm bath of her famous charm ("Oh you're so tall! I love tall girls!") it really is like being caught in the chill wind of an icy draft. Major Lumley was, by all accounts, a softly spoken man, with matinee idol looks, who fought in the vicious guerrilla wars of the Malaya Emergency. And Lumley is a chip off the old block.

After her triumph with the Gurkhas, there was a lot of talk about Joanna Lumley for PM, or at the very least, as an MP (and she did seem to take some interest in politics, funding Caroline Lucas's election campaign for the Greens). But of course, she'd make a far better queen. She'd be magnificent as Boudicca, although in her new play, she's actually playing another queen: the lesser known, but still powerful, Eleanor of Aquitaine. It's a Trevor Nunn revival of a 1966 Broadway play The Lion in Winter, which was later turned into a film with Katharine Hepburn playing Lumley's part. And if anybody is capable of taking on the role of the scheming, brilliant Eleanor, who marries first the King of France, and then the King of England, it's Lumley. Though she says she hasn't played a queen since she was seven ("My first role, it was the AA Milne poem, where the king asked the queen and the queen asked the dairy maid, could we have some butter for the royal slice of bread?").

Still, she's met a fair few. "Having had the great honour not just of meeting our Queen, but also a few other queens, the Queen Mother of Bhutan, and Queen Noor of Jordan and a few others, it's just a title. They're just people. And that's how we are in the play. We've got two kings in it, Henry and Philip, we've got two kings-to-be, Richard the Lionheart and John, and we've got a prince, and me, a queen, and another princess. So we're all royal. The entire cast is royal but they're also just completely ordinary. There are no servants. No bowing and scraping. We're just a family."

Just a family where the husband has imprisoned his wife and they're all plotting to decide who'll be the next king of England, but you get the point. It's a terrifically meaty role, for which Hepburn won an Oscar, though Lumley says she hasn't watched it. "If that sort of thing bothered you, you'd never work. Everything's been done before. Although I would think twice about doing To Kill a Mocking Bird with somebody other than Gregory Peck. I would be careful, oh young actors, if you think you can take that one on. Sometimes you go, 'That. Was. It.'"

Do you ever watch somebody else playing a role though and think: I'd love to do that?

"Yes. But only if they do it badly. You think: Well I could have done a better job than that. But if it's very good, you think: Ooh, that's peachy! I'll remember that one. That's gorgeous!"

There's a touch of Enid Blyton to Lumley's phraseology. She has a distinctive turn of phrase, and employs the kind of adjectives that if they were sold on eBay would be called retro chic. But she's also sharper than the 50s lingo might lead you to believe. Particularly when she doesn't think much of the questions. We talk about the theatre and the fact that she's said she doesn't like long runs.

"There's not an actor in the world who'd disagree with that. Well, would you? Would you like to interview me, every day, eight times a week? Would you? Would you? You'd have to. So think again. How are you going to ask these questions with a new fresh mind, fresh again. Fresh! Again! To me. And be interested in the answers. Because that's what we do. We don't just spool it out. Do you think you'd enjoy that?"

Hmm. Well, not so much, perhaps, when you put it like that. Though I don't doubt that she manages it beautifully. She's such a pro. An uncomplainer. She's just brought out a new autobiography, Absolutely – a photo book of her life fleshed out with a bit of text – and there she is in her model days, moue-ing for the camera, and trussed up as a dollybird for some terrible British sex comedy (not that she complained – it got her her Equity card), and then there's her Bond girl moments, and the Purdey years, her breakthrough role in The New Avengers when her pudding bowl haircut was the acme of 70s chic, copied by hairdressers throughout the land, and the great sci-fi series, Sapphire & Steel, ITV's answer to Dr Who.

The photos are a treat. There are knitting patterns and miniskirts and kaftans and afro wigs. As well as some of her 1970 marriage to Jeremy Lloyd, an actor and scriptwriter who went on to write Are You Being Served?, and which lasted only a matter of months. "It seemed to have a bit of an Austin Powers quality to it," I say.

"Absolutely! It was shagtastic. It's lovely when you look back at old photos and go, 'Oh my God!' We really did dress like that."

Then in the 80s, it all goes quiet for a bit – roles in TV dramas you can't remember or haven't heard of. And then, finally, there's Patsy. Patsy was the role that Joanna Lumley had been waiting all her life for, a grotesque comic creation who leaped on to our screens, drinking and smoking, pouting and snarling, and introducing middle England to hitherto unknown concepts such as Harvey Nicks and lashings of Bolly. And now, she's back. Jennifer Saunders has written three new episodes of Absolutely Fabulous which will be shown this Christmas, and the world will get to see how the years have treated Patsy.

Were you nervous about reprising it?

"It wasn't my choice. The BBC said it'd been 20 years since the first one. And we were like, 'Whaaat?' And when we all walked in, with Julia [Sawalha] having been huge in Lark Rise, and Horrocks, who never stops working. And Whitfield, who's the only person I know who's eightysomething and always in work. And Jennifer, who's done all sorts of things. We all came in from different places saying, 'Wooo! Here we all are again.'"

Was it exciting?

"It was lovely. It was quite strange. You take a bit of time getting back into your person again."

But surely Patsy's never left you?

"No, Patsy's never left me. But you only do what is written. So they'd moved it along a bit. Life has not left them behind. They're getting older. Although I wonder if I'm deluding myself, because they all look exactly the same young women that they did 20 years ago. Apart from June, who really never ages."

I'd read an article, which made the point that Ab Fab presaged the great age of PR and our modern-day obsession with brands. That there's nothing left to lampoon: we're living in an Ab Fab age, where the prime minister is a PR man and his wife runs a luxury-goods firm.

"I know all our old targets have changed. In the beginning, the Princess of Wales was still alive. And the Duke and Duchess of York were still married. But Jennifer is smart. She's bloody brilliant that girl, so she isn't writing it like episode one. She's writing Ab Fab for today."

Is the modern day equivalent of having a Buddhist temple in the corner of your living room having a wind turbine on your roof?

"Yes! Exactly. Edina's great passion is that she's always changing. It's always the same house but it's always changing."

And then, it all starts to go ever so slightly wrong. Am I right in thinking that Ab Fab was your comic entree, I ask. That people hadn't really thought of you as a comedian before?

"The knives you're sticking in, Carole! I don't know why you've got it in for me like this?"

Eh? I think she's joking. Or hope she is. I'd meant it as a compliment. That the world had failed, until this point, to recognise her comic gifts. Because her timing is brilliant in Ab Fab, her delivery perfect. Twenty years ago, it just so happens I was a waitress at the press launch of Ab Fab. And it was obvious to everybody there that the BBC had a hit on its hands. And that Lumley was the star. I didn't even see the show, then (too busy pouring wine; it was a thirsty crowd). I just saw the journalists pressing around afterwards to ask questions. They were enthralled by her. The sex goddess who'd suddenly grown a potty mouth and a vicious sense of humour.

I never get the chance to mention this, though. "I'd done stuff. I'd done comedies. I'd done stuff."

But you were better known for drama?

"For the parts I was cast in. I now can't even remember what they were. Whatever you're in, people go, oh… I mean we've got Robert Lindsay in this play who's a fabulous actor and people go, 'Isn't he in My Family?' And you go, 'Oh… OK. Yes, he was.' And yes. Yes, I was Purdey. Which wasn't a comedic role. So. No."

Did you enjoy being given that chance?

"Of course I did! Those scripts were so good. Anybody would. God. It was like being given a glass of champagne. You would just read them and go, 'Oh wow, now this is in a different league. This is something else.'"

She's such a grotesque, isn't she?

"I made her. She wasn't written like that. Not in the first episode. We invented her. And she got stranger and stranger and worse and worse. She was initially written as just a friend. But like anything, when you're given a part, you make the part. Jennifer knew her part because she'd played it with Dawn and Dawn had played Saffy in the original sketch. So, she'd had the idea of how those two would work, but she hadn't got a mother, she hadn't got Bubble, she hadn't got any any of these other people. Who were invented."

It struck me that Patsy is almost the anti-Joanna, I start to say, but Lumley interrupts. "Well, she's not. Jennifer is convinced that I am Patsy. In that she's eerily like me. Or at least we've eerily, and on purpose, invented a lot of stuff that is rock solid in my life. Patsy had been a model in the 60s, and so had I. And that's easy because I can remember and furnish her with all that stuff."

Usually, actors go on about how they're not like their most famous part, so it makes for a refreshing change. But perhaps it's the Patsy in Lumley that's helped to make her such a one-off. Such a force. When I ask her about the backlash against her and the Gurkhas – it was reported last month that a Facebook page entitled "Joanna Lumley has fucked up Aldershot and Farnborough" – had amassed 2,500 followers, she's diplomatic, at first. "It's quite hard for me because I live in Stockwell [south London], which is very much a multiracial, multicultural, multi-religious, multi-age, multiethnic place, and I adore it, which is why we live there. So it's always a bit of a shock when you discover that some people can't bear other people. Are frightened of different people. Particularly as the Gurkhas, and the Nepali community as a whole, is incredibly generous."

The complaints from the locals were that the sudden influx of Gurkhas had put a strain on local services. And people couldn't understand why Lumley, who had once been so vocal, had become so quiet. But eventually it comes out. The government had asked her to back off, she says. They – Lumley and the Gurkha campaign – knew there would be problems, which is why they'd asked for certain safeguards to be put in place. "We knew this was going to happen. And we said this information must all be sent out to Nepal. They must be all be alerted before they set off. Fuck off, we were told. We said it must be written in Nepali as well as English. We said when they arrive here, we must be absolutely certain that there is a network of people here. They said, 'all that is our business'. So we said, 'OK, you do it', and sat back and watched none of it being done… It's gone wrong absolutely as we predicted it would if they didn't put these failsafes in place. And guess what? They haven't been."

She always seems to have known her own mind. There's an old clip of her on YouTube, on Parkinson, in the mid-70s, in which Parkie asks her about being a single mother. Lumley got pregnant, aged 21, and became an unmarried mother at a time (1967) when it was still a disgrace rather than a social norm. It's a striking clip, because Lumley is so articulate, and sensible, and forthright. Being a single parent, she says, is not a moral problem, "just a practical problem". Even more striking is just how different it is to one of the same era, Helen Mirren, also on Parkinson. Back then, long before DCI Tennison, Mirren was a flirtatious ingenue with a voice almost as posh as Lumley's. She's changed utterly. Whereas Lumley seems essentially the same. I wonder if she might have got better parts if she too had toned down The Voice. But I don't get the chance to ask, though it's Lumley who later brings it up.

Instead, I ask her about the book. It's so cheerful and upbeat: her family are marvellous, her first husband, lovely, her second husband (she married composer Stephen Barlow in 1986) lovelier still. Being sent away from her family in Malaya, to boarding school in England, aged eight, was "cold" but not terrible. Getting accidentally pregnant brings out her parents at their supportive best. "Relentlessly cheerful," is how I describe it. Which, in interview terms, is what we would call "a mistake".

"I could write a very different book. Very, very different. And I would never write it. I would never write it! It would be hurtful. It would be cruel."

So, this is only one version of you, then?

"What do you think? What do you think, Carole?"

I think that everybody probably has several versions of themselves.

"I've written two more books. You know that? Both, as you say, relentlessly cheerful. But with curtains showing the darkness. I'm not going to go into the darkness, but it's there. I'll show you where it is… But I can only celebrate the fact that I have a child, that I'm adored by my parents, loved by my sister, adored by my friends, doted on by my baby. What's to be sad about? Why do people go on whining? Why? Why? Have you seen a woman with four children she can't feed and one baby dying? How can you go, 'Oh, I'm so unhappy.' I'd never read those books. Never. They make me sick.

It's her family, she says, who instilled in her a sense of right and wrong. And they do seem a remarkable bunch. Her grandfather was a close personal friend of the 13th Dalai Lama; her grandmother the first European woman to enter the Potala Palace in Lhasa. It's why she's so passionate about education. "The only reason people know anything is because they're taught it. The only reason I wasn't part of those riots burning streets was because I've been taught not to do that. Otherwise I'd be in the shops nicking, but I was taught not to do that. So I don't go and steal things."

It's an arresting image, the idea of Lumley out rioting. But there's no time to ponder it, because she's off. Riffing about her good fortune, and the bad fortune of others. "You think there's something wrong, or grotesquely one-sided about me. But I'm really not self-pitying. I've nothing to pity myself for. I consider myself lucky. And I know I must be unbearable and to have this voice! That I never listen to, but people go, 'Oh posh! Oh, plummy!' And you go, 'Am I? Am I?' What if I had a Somerset accent? Would you find that grotesque and mock that? I don't know what I sound like. I have no interest in myself. I just know that I'm lucky! Luckier than perhaps anyone on Earth. And to write a misery memoir, why would I? I have no misery in my life. My only misery is the injustices in the world that I see. Which is why you gird up your loins every day, and go, 'Woo! I'll have a crack at something.'

There's no stopping her now. I try to interrupt. But she'll not be interrupted. Joanna Lumley still has an awful lot of things to say.

"I'm so astonished by cruelty when it comes. You think: Why are you doing that? So when things come up out of the blue, when some cruelty or some abominable act like the bankers giving themselves bonuses… I can't believe it! I can't believe you boys. You! Boys! You! You are accepting money that we gave you. And look at her. She can't even afford heating! She's bailed you out. And you're giving yourselves a prize? Where's your fucking shame? And he just goes, new car for me. And the Sunday Times rich list goes 'Wooooo!' And in the rich list are people who are really rich! And you go, 'You deserve a tight slap.' Why do we adore riches? Why? We think riches are power. We think riches are marvellous. We do in our our country, we do. We adooore money."

It's perhaps as well at this point that the PR walks into the room and the time's up. I'm worried about Lumley's heart. She's just so ferociously engaged with everything. On Graham Norton, two days later, she says she starves herself all day and then has a large meal at night. I admire her fighting spirit, I really do. I just think the odd bowl of porridge might not go amiss.

The Lion in Winter runs until 28 January 2012 at Theatre Royal Haymarket, London SW1