Joanna Lumley glides into the screening room. "Hallo, lovely people!" she purrs at the group of journalists assembled to view her new TV documentary, Cats, and to pose a few questions to the woman famed for being Purdey, Patsy and, more recently, Goddess of the Gurkhas.

In dark trousers, blue top and elegantly swathed scarf, Lumley sits at the front of the room smiling down beatifically at the arc of Dictaphones set out before her, answering questions about every kind of cat. She jokes about the cat rock band she met in Chicago who play the drums in return for salmon, and about the cat-owners who dress their pets as frogs, cows and mice. She seems fantastically good fun.

Lumley's last cat was a handsome tabby named The Bee, who died some time ago. "The Bee was heaven," she says, in a warm, sad kind of way. We have moved to a quiet room set out with a coffee table, sandwiches and cakes and fancy yoghurts in long-stemmed glasses. "He thought he was a dog, I think, because when I used to go and post letters in the village where we lived down in Kent, he would walk down the village street with me. Right up to the post box, then walk back again." She smiles. "And he was a great companion cat, so he would always come if I was gardening or if Stephen [Lumley's husband, the conductor Stephen Barlow] was clearing stuff or washing the car. He would always sit, and watch, and be with you." She had no hesitation, therefore, when ITV approached her to present a documentary series about cats. "Leaped at it!" she declares. "I was very jealous of Martin Clunes, having done the dog one." Lumley makes this kind of statement with such effervescent charm, like the perfect hostess complimenting your dress, that it is only afterwards you wonder if it was true. This is of course part of her enduring appeal, a kind of jovial bewitchery, and a voice that is soft and fragrant and as perfectly English as gin.

How, I ask, attempting an awkward segue into non-feline matters, did it feel to have recently been declared a goddess by the people of Nepal, following her successful fronting of the Gurkha Justice Campaign? "Well, it's beyond comprehension actually, you know?" she says modestly. "Somebody explained to my son Jamie who was out there, that in Nepal they believe that if the gods can't handle something, a problem, they pick somebody out and send them to earth to solve the problem, and the person who solves the problem is called a god or a goddess. And because my head was above the parapet, because my face was the identifiable one of our team – we were a team – they maybe saw me as that person who had been sent to sort the problem."

Not a frightening god, mind. "I don't have it in me," she says. "I don't lose my temper. I used to, but I realised I would probably die of a brain haemorrhage. So I've governed myself not to mind about things. I have no road rage or anything like that. Because it's life-shortening. And also there's no need for it; it uses up energy. I don't mind not eating, or sitting in the bad part of a restaurant or being snubbed, it doesn't really matter to me very much. So I don't think I'm much of a god, because I don't have the good snappy nature that would throw a thunderbolt."

Instead, Lumley has learned the subtle art of persuasion – to which anyone who witnessed her confrontation of immigration officer Phil Woolas would attest: at the BBC Westminster offices in May, Lumley, incensed by the rejection of appeals for residency by five Gurkhas, pursued Woolas around the studio and forced him into an impromptu press conference, in which he agreed to further talks on the matter. The BBC's political editor Nick Robinson declared that he had rarely seen such a scene in his whole career. "Well," explains Lumley, gently, "with Mr Woolas we had just come from a room where we had been talking with the lawyers and [Liberal Democrat] Peter Carroll for about half an hour about what the next step would be, and in that press conference all I was doing was saying aloud and corroborating with him what was said. But it looked a little bit as though I was giving him a tough time. So to make up for that I did invite him round for supper." Did he come? Lumley smiles slowly, broadly. "Yeah," she says. "And he brought two friends and we had fish and chips and champagne by candlelight."

There is, she argues, a logic to taking a more nuanced approach to browbeating. "The thing about water is it's strong," she says. "And a river running down, if you dam it, it'll break the dam. So find a way round, but don't stop something. And if you can find a way round it's usually much better. And people don't get hurt – there's no reason for that. And people don't lose face, which is terribly important. But if a change of mind can take place in their own minds, rather than being forced to it, I think it's the best way to do anything." She pauses. "I'm a vegetarian and I long for people to eat less meat," she adds, "but the thing to do is not to go, 'Eat! Less! Meat!" It's to say I am fit as a flea and I'm 63, I haven't eaten meat for 40 years, and I never get diseases, I'm never ill, and I'm full of energy. So how's about that?"

It's an approach she claims to have learned from her mother's father, a diplomat. "I love diplomacy," she says. "I love courtesy and kindness." She describes her huge disappointment at Britain's failure to pursue a diplomatic solution to the escalating problems in Iraq. "My father, having been a soldier, said: 'You should do everything on the planet before you go to war.' It is such a dreadful thing to do. War is so indefensibly dreadful. All frontline soldiers know that, know how utterly ghastly it is and that you must never go there again light-heartedly. So anything that stops conflict, I'm all for that."

It was Lumley's father, of course, a Major in the 6th Gurkha Rifles, who inspired her campaign. "Soldiers are necessary," she says. "And the thing about the Gurkhas is that if things have to be done, they just do it without any kind of malice. They're just utterly efficient machines for doing what they do. Machines is the wrong word, but . . . It seems to me a good thing to do, a brave thing to do, to protect people. I have the highest regard for people who guard us, which is policemen and the armed forces."

Earlier this year, as the country was afflicted by Lumley-fever, as there were cries of, "Lumley for PM!" and the people of Nepal named a mountain in her honour, there were still one or two detractors, those who claimed that allowing the Gurkhas UK residency would deprive the already poor nation of Nepal of the financial support provided by the soldiers' pensions. Lumley rolls her eyes a little at the mention of it. "Yes, we've checked that out," she says firmly. "It's going to be far better, farrrr better . . ." she stretches the word like a piece of elastic. "If any of them work over here they'll make far more than they get on a pension. All the Asian communities here send money back home.

Lumley has spoken before about the fact that she has throughout her career played good people, from the crime-fighting Purdey to the Bolly-swigging Patsy, via a brief turn as Ken Barlow's love interest in Coronation Street. Add this to her Gurkha campaign and the 60 charities she supports, from Wateraid to Tibetan refugees, orphanages and schools, supporting the Pastoralists in the Horn of Africa and the Born Free Foundation, and you begin to sense that she is acutely aware of the importance of being good.

"God yes!" she says at the suggestion. "Terribly easy to be bad! It's easy-peasy to kill something or break it! I've never been remotely impressed by people with guns killing people in films, it's nothing to me. Of course you can kill people! I could get a gun out and shoot you dead now, you're dead, it doesn't make me powerful, it's just completely stupid." Indeed two years ago, she legendarily confronted a gunman in a Sheffield bar, engaging him in polite conversation until the police arrived. "Being good, however, is fabulously hard," Lumley continues. "And we all fall off at every fence, you know? And there's nothing wet about being good! I think sometimes there's something quite wet about being bad!"

Badness, she explains, comes in various degrees. She rumples her brow. "I hate the hand that comes out of a car and just drops litter in the street," she snaps. "I hate that! For some reason it just fills me with fury! It's just utter laziness, lack of interest in other people, lack of interest in the planet, in the hedgehog who might eat the plastic bag, it's a lack of concern." Would she tell someone off? "Yes, but not horribly," she says. "I would package it up and say, 'I believe you dropped this.'" Her voice is buttery. "And if they were horrid I would think of a different way of dealing with it." She pauses, smiles, leans a little closer. "I've got to tell you in service stations on big motorways I always clean up the ladies loo. I pick up all the bits of hankies, I tidy up the bins, I get using the towels, I clean the tops, I shut the doors, I pull the plugs . . . Because people live like animals. And surely if it looks nice people won't go on making it look so bad? If you walk into a midden of filth maybe you just don't care about it? But it does baffle me how people can behave so badly."

It is perhaps this deep-rooted belief in goodness, along with a pervading sense of sleeve-rolling practicality and her glamorous fearlessness, that make her such a fine documentary host. There is something to be said, too, for that famously mellifluous voice. "I don't know," she says lightly. "If you're in something you don't ever hear it. And I'm lucky, believe me, I'm lucky. I get as many offers for drama as I do documentaries." Earlier, in the screening room, she had dismissed talk of ageism on screen, arguing that women of her age shouldn't expect to get the roles of a 23 year old. "I was never offered them," she says now. "I mean I was 30 when I got Purdey. So I was never playing Juliet at 14."

Not that she would have wanted them anyway. "I've never been interested enough to have a career trajectory," she continues. "I've never had any ambition, or thought of what I should be doing or had any idea of what I'd like to do. Never. And still don't. And if something comes along I say fine. It's like food, I like it," she waves at the spread on the coffee table, untouched, "those sandwiches there, they look fine, I wouldn't have ordered them, but now that they're here they look fine. So that's how my acting is." She stops for a moment and looks at the sandwich platter. "But if I had been a raging beauty I would have gone to America I imagine, and made a career out there. Because you can't do that over here; this country slightly despises beauty, and so all our lovely, lovely ones go and make a hit of it over there. The Catherine Zeta Joneses and the Kate Beckinsales."

It comes as something of a surprise to discover that the much lusted-after Lumley shouldn't consider herself a raging beauty. "No ,never! And never was!" she insists. "I must tell you this. But I had a useful face. I could work with it – like my hair, it's not nice but it's useful, it can be dyed different colours, go up and go down. And so I always saw myself as a character actress, which anyone in their right minds would rather be than a beauty actress. Beautiful is very boring."

Still, that useful face of hers, that voice, that charm, have brought an enduring appeal for Lumley, who has remained for more than 30 years one of the most popular faces on British television. She bats away any suggestion of a particular beauty or talent. "[Programme-makers] know that I'll be a good old team person," is how she explains it. "The fact that I'd been a model in my early days, the fact that I can put my own face on and straighten my own hair back, drag out some crumpled bit of clothing and pretend it looks all right. So I'm all right as a team player," she says. "And I'm interested in things," she adds, "always interested in things." So, I ask, if anything should ever happen to David Attenborough, would she be prepared to fill his shoes? "Oh," she says softly, "nobody could fill his shoes. But he can have my shoes. Even the shoes I made out of a bra" •

Decca Aitkenhead is away