Is she, Miriam Margolyes would like to know, how I expected her to be? She is. “Everybody says that,” she replies, a little sadly. But isn’t it good to go through the world being utterly yourself? She smiles. “I know, but it would be so nice to surprise people.” Even if you know what to expect – anyone who has seen one of her regular appearances on The Graham Norton Show knows how gloriously naughty she is – Margolyes, up close, is still quite something. I point out to her that we have been talking for only 10 minutes and already she has answered the phone (to a child, she later finds out) with a terrifying, booming: “Who are you?”. Meanwhile, I’ve spotted the remnants of a chewed onion on a plate, which she ate, like an apple, for lunch (“You mean you don’t?”). She looks pleased.

There is something bracing and delightfully scandalous about Margolyes. It’s not an eccentricity that has grown with age – she has always been like this, it seems. According to the actor Tony Robinson, in one of her first acting jobs she turned up late to rehearsals, apologised and said she had been “wanking off” a park-keeper. Why does she like behaving so badly? “It is true that in shocking people, there is a kind of delight in people’s: ‘Did she say that?’ I think giving people a bit of a jolt is good.”

I meet Margolyes in her flat in south London, which looks out on to her lovely garden. She is 78 and shrinking, she says – she has lost an inch and is now 4ft 10in – but there is nothing diminished about her. Her career is marching along nicely – her forthright Mother Mildred in Call the Midwife has brought her to primetime TV, and her role as Professor Sprout in the Harry Potter films made her famous with a new generation, even though she has said she doesn’t like children very much.

Her force-of-nature personality – her empathy, warmth and hilarious openness – has been perfect for her new direction presenting documentaries. Her latest, Miriam’s Big Fat Adventure, for BBC Two, will be a two-parter looking at the obesity crisis and the body-positive movement and exploring her own relationship with her weight.

“I’ve been fat all my life and I wanted to work out why,” she says. “Could I change? And how do other people cope with it?” Being overweight “has affected and infected every part of my life. It made me less attractive, sexually. It made me miserable often as an adolescent.” It also made her develop her sense of humour, “because you can’t go around miserable all the time, so you have to make people laugh. I think it made me quite aggressive on occasion because I won’t be bullied.”

Miriam’s Big Fat Adventure. Photograph: Wild Pictures/BBC

She hates being overweight, she says. “I look at myself and I feel distressed, anxious, disappointed, envious of people who aren’t [overweight], annoyed that I can’t get clothes to fit.” She was surprised, when she met some gorgeous, empowered women of the body-positive movement, to find that people could be non-skinny and happy. “I never really believed that until I met these people,” she says, eyes widening. “I admired them. I wished I could be like that.” Making the programme has made her less harsh on herself, she says. “I think I’m much more compassionate about myself than I used to be.”

Margolyes started putting weight on when she was about eight or nine. “By the time I had my periods, when I was 11, I had a 36in bust, which was the focus of much attention from the American soldiers around the place.” I must look alarmed because she adds: “I didn’t mind, I thought it was wonderful.” The alarm on my face increases. “I was quite adept at saying, oh you know, “silly” or … because I wasn’t interested in it. It didn’t inflame me, but it certainly inflamed them.” The phone rings and she barks down the line to the young boy who has called – his family is staying in her Kent holiday cottage – checking he has her autograph, telling him an actress whom he almost certainly hasn’t heard of (“I don’t like her”) comes from his town, before finishing with: “Look, I have to tell you, I’m in the middle of an interview with a very important newspaper called the Guardian. Because I’ve got a television series which is coming out. So make damn sure you watch it.”

At the end of the documentary, Margolyes looks emotional when she talks about how she would like to feel about her body. But what seems surprising – because it’s unusual to hear women talk like this – is that she also says she loves her face. “I think that when people look at me, they smile, and they don’t smile mockingly, they smile affectionately,” she says now. “And I love that.” It doesn’t bother her that she has never been a conventional beauty, even in an industry where that is prized. “I don’t want to be a conventional anything!” she says. It sometimes bothered her as a teenager. “I used to feel sad that nobody used to ask me out; I felt very crushed by that, actually, but it didn’t last. And when I went to university, I realised that I had a spark of something that was more valuable than beauty. I had energy, and energy is always attractive.”

Margolyes always knew that she was “a performer, and I love an audience”. She grew up in Oxford, the cherished only child of Joseph, a Scottish GP, and Ruth, who developed property (she rented student houses to Ken Loach and Tariq Ali). She describes their tight-knit Jewish family as “a fortress, just the three of us. It was passionate, close, adoring.”

As the Spanish Infanta in Blackadder. Photograph: BBC

She loved doing public speaking competitions, which she always won (“That was how I first realised that I could entertain, and that was very pleasing”) and at Cambridge she joined the Footlights theatre group, where peers included John Cleese, Eric Idle and Tim Brooke-Taylor.

After university, she sold encyclopedias and worked for a market research company for a couple of years while trying to make it as an actor. “I didn’t get auditions very easily because I hadn’t been to drama school,” she says; it was getting cast for a BBC radio drama “that really set me going”. She has had a hugely varied career. She was in Barbra Streisand’s Yentl and Baz Luhrmann’s Romeo + Juliet, appeared in Blackadder in the 80s, won a Bafta for her role in Martin Scorsese’s Age of Innocence and has many acclaimed theatre roles under her belt, including her one-woman show Dickens’ Women.

Margolyes came out to her parents when she was 27. It was the late 60s, and unusual in those days to be so open. “It was very unusual, I think, and very foolish,” she says. “Ian McKellen, my dear friend, and I have this constant difference of opinion because he feels that you should come out [to give encouragement to others]. And I say, it depends who you’re coming out to. It hurt them too much and it didn’t please me particularly, so I think it was an error.”

A few months later, Margolyes’s mother had a devastating stroke; Margolyes has always believed that her coming out in some way caused it, and she spent the next few years helping to look after her mother. “It was a terrible time,” she says. “I was relieved when she died. I was in hysterical grief, but I am glad that it happened because I couldn’t have taken much more – when you see somebody you love unable to speak or think or move, you’re torn in bits. It’s a terrible thing.”

With John Castle in the Tales of the Unexpected episode Fat Chance. Photograph: ITV/REX/Shutterstock

Margolyes didn’t have her first girlfriend until she was 27. Had she known she was gay from an early age? “No, I don’t think I had. I think it all came as a bit of a bombshell. I used to have crushes [on women] and they were incredibly passionate and all-consuming. I wasn’t really interested in men and I’m still not – I’m not interested in their souls, because very few men have souls. When you find one that has, it’s worth it. Most of them are just so trivial. And if you’re not sexually interested in men, they’re unbearably trivial.” Maybe she’s just meeting the wrong men? “It could be. I can like them and have my male friends, but I just don’t feel groin excitement from them.”

We talk about the television presenter Philip Schofield, who has just come out at the age of 57 (Margolyes has appeared on This Morning several times, most recently in January when Schofield and his co-presenter Holly Willoughby had to apologise for Margolyes’s use of the word “twat”). “Poor darling boy,” she says. “There are millions, unfortunately,” who feel compelled to lead a secret life. “We live in a bubble of people who don’t care what sexuality people are. But there are a great many other people in the world out there who do care and make life miserable for people because of it. And I think it’s awful.”Was her sexuality ever something she struggled to accept? “I was thrilled!” she shouts. “Ecstatic. I think it gives you an identity because you can say” – she puts on a grand voice – “‘I am a lesbian.’ It put everything into place. And it was fun. I still think it’s an absolutely terrific thing to be, which is good, because it’s not going to change.”

But she must have been very hurt by the way her parents reacted. “It was terrible,” she says. “And I was very, very unhappy. I knew I couldn’t change what I was; I was just unhappy that I told them. But I told them because my relationship with my mother was completely open – we had no secrets from each other.” Was she not angry with them for not accepting her? “No. They came from a world which could not adapt and that’s sad. It’s their tragedy, but it didn’t become my tragedy. I’m lucky that I’m not …” She takes a rare pause. “I’m remarkably un-bitter, really.”

Margolyes has been with her partner, an Australian academic, for nearly 52 years. What is their secret? “I think love and trust, and telling the truth. Never let the sun set on a quarrel. We don’t live together, which is probably why we’ve stayed together that long. She likes to work and I do, too. Communication – you must talk to your partner.” Her partner lives in the Netherlands and they see each other only about eight times a year. “I speak to her every day on the phone, sometimes more than once.” They are currently talking about whether to live together. Would their relationship work in close proximity? “We don’t know; we’d have to find out. I think so, because we really do love each other. I love her more and more.” She smiles. “I don’t know that she loves me more and more.”

Do they still fancy each other after all this time? Margolyes smiles mischievously. “I don’t get wet when I look at her, if that’s what you mean,” she says. “I don’t feel physical lust, but I feel mental lust. I love talking to her, and I suppose my happiest moments, really, are just lying in bed with her and looking at the ceiling and talking. That’s my big joy.” Anyway, I say, to stay with somebody (and, I think to myself, somebody who regularly eats whole raw onions) for so long is a big achievement. “Yes, I think it is,” says Margolyes. “But I am a remarkable person.”

In the 1993 TV film Ed and His Dead Mother. Photograph: ITV/REX/Shutterstock

Margolyes has always been political – and is more so now than ever, I say. “In this climate? I should think so. I’d rather not have to be, but what a collection of nasty people there are in charge of us. What a disgrace. I’m disgusted by that government.” She is, she says, alarmed by the rise of rightwing nationalism, and by the anger and hatred she sees on social media. “I know that we could have a pogrom easily in this country. It could be against Jews, it could be against Poles, it could be against Gypsies. There are people who are boiling with hate and resentment and entitlement in this country and it is alarming.”

She has been vocally critical about the Israeli government over the years; she says it has lost her many friends. “It’s because I say things about Israel that I consider true. If I think something is wrong, I will say it, whoever is doing it.”

She was a supporter of Jeremy Corbyn. How did she feel about the allegations of antisemitism in the Labour party? “I think that there is some antisemitism, but it is nowhere near as much as people say. And I think there’s plenty of antisemitism in England. People don’t like Jews. I’ve accepted that.” Antisemitism, she believes, is “widespread in the Tory party, but that’s never mentioned. Corbyn handled it badly and I regret it, because he’s a good man and he’s not an antisemite. But he should have gone, he should be out the way and let’s get on with new people.”

Her wrath is reserved for the Tories. “I love British culture, I love the theatre, I love literature and the arts, the countryside, and I just feel that all the beauties of the place are being corrupted by this evil government.” She is horrified by the attacks on the BBC. “It’s outrageous. It’s vindictive,” she says, her voice rising. She cannot stand Boris Johnson. “He’s like Trump. He’s a ruthless narcissist, dangerous, drunk with power. The fact that my European citizenship has been taken away from me by that crowd of charlatans and rascals is absolutely appalling. It’s the central most miserable thing in my life at the moment.” She could always move to Australia (“wonderful country”), where she has a house and citizenship, but has ruled out living there permanently. “I will try as long as I can to have two lives. Life’s like a cream bun, you know – you want to have as much as you can.”

How does she feel about getting older? “Irritated,” she says, although she concedes that people listen to her more. “They don’t see you – you’re invisible when you’re in the street – but if they’re sitting talking to you, they think you’re going to be talking sense because you’re old. And it may not be true.” She says she demands respect. “When I get into a tube train and I want to sit down and there’s no seats, I say: ‘Please, may I sit down?’ And if nobody will get up, I sit on them.” How many people has she sat on? “I think actually just one. They were surprised, and then really quite angry.” She looks delighted.

Miriam’s Big Fat Adventure starts on BBC Two on 9 March at 9pm