It’s the 70th edition of the Cannes film festival; a birthday to celebrate, an excuse to look back. The list of past Palme d’Or winners scrolls across the screen ahead of the evening premiere. The whole town is studded with pictures from yesteryear. There’s Brigitte Bardot posing on the beach; Godard and Truffaut rallying the masses; Blow Up-era Vanessa Redgrave in a striped mini-dress, the epitome of swinging 60s London. “Oh yeah, very glamorous,” the actor recalls. “I came with Michelangelo Antonioni and Monica Vitti, so I felt I was really in with the cool crowd. Except that we didn’t call it cool in those days. I’m not sure the word even existed back then.”

Fashions shift and time marches on. And Vanessa Redgrave turned 80 this year. She comes picking her way across the hotel garden like a waiter carrying a tray of precious china. Her eyes are bright behind her oval specs; her features as sculpted as those of some beautiful wood statue. I’m tempted to view her as a piece of living Cannes history but suspect the very idea would enrage her. She’s arrived in town like a Fury; she’s on urgent humanitarian business. So she orders an espresso and promptly sets down to work.

Specifically, Redgrave is here to promote the documentary Sea Sorrow, her belated directing debut. Prompted by the 2015 death of three-year-old Alan Kurdi, the film is a shonky but impassioned cri de coeur, spotlighting the plight of the migrants stranded outside Calais and damning the UK government’s efforts to repel them. But this is also, tangentially, Redgrave’s story as well, in that it details her political education in the wake of the second world war. Sea Sorrow (the title taken from The Tempest) hails the Universal Declaration of Human Rights adopted by the UN General Assembly in 1948 and then lambasts the way its rulings are now being ignored. It suggests that the UK has mislaid its moral compass, drifted away from core principles.

“Not drifted,” she corrects me. “Violated. It has violated these principles and it continues to do so, which I find deeply shameful. The UN signed the Declaration of Human Rights and now we have to employ lawyers to take the government to court to force them to obey the law. Just thinking about that makes my mind go berserk.”

“Vanessa, you see, is here on a mission,” explains Alf Dubs, the Labour peer, who is sitting at the next table. It was Dubs who, aged six, fled Nazi-occupied Prague on the Kindertransport; Dubs, too, who spearheaded an amendment to the Immigration Act 2016, ensuring unaccompanied refugee children safe passage to Britain (a scheme since abandoned by the Home Office). “But it’s fantastic to have Vanessa as the driving force,” he says. “She adds a dimension that we in politics just haven’t got. I’m seeing her as the carrier of the torch.”

I want to talk about Redgrave’s life and career but she is insistent that the film takes precedence; there are vital issues at stake. So we sip our espressos and discuss the rights of the child and the destruction of the “Jungle” camp in Calais and how we live in a dark time that she truly hopes will improve. At one stage in the film, she likens the current era to history plays such as Richard III; the same wanton bloodshed; similarly monstrous main players. I’m guessing she’d cast the likes of Trump and Putin as grotesque Shakespearean villains.

“Well, I’d use the word Shakespearean. Grotesque is your word. I try to be more measured and precise. I mean, I can think of worse epithets for those people you mention, but that’s like Hamlet saying, ‘Who am I to curse?’. You have to go deeper than cursing if you’re making a film.” She pauses to consider. “Yes, some films can curse, but that wasn’t my goal. What I wanted to do was to have people see this film and see these refugees and think ‘that’s one of us’ as opposed to ‘that’s one of them’.”

Out of the blue, she becomes mildly vexed. “Can we talk about the film and not me? You keep trying to bring it back to the personal.”

Oh, come on, I splutter. I’ve barely asked you anything personal at all. In fact, I was just about to come to that…

“Can I get you another espresso? To revive your flagging spirits,” Redgrave cackles. “You see what I’m trying to do here. I’m directing you gently away from the subject.”

Too bad, here it comes; there’s too much history to ignore. Redgrave was born into a theatrical dynasty, the daughter of Michael Redgrave and Rachel Kempson. Laurence Olivier announced her birth on stage at the Old Vic and predicted she would go on to be a great actor. Actually, she confesses, her first love was dancing, but she gave that up when she grew too tall. On screen, she has played Mary, Queen of Scots and Anne Boleyn; Guinevere and Mrs Dalloway. Except that her sister – the late Lynn Redgrave – reckoned that she “always thought of herself as Joan of Arc – a touch of the martyr”. Undeniably her best roles contain a kind of agonised intensity. I’m thinking of twisted Sister Jeanne in Ken Russell’s The Devils, or Julia’s doomed anti-fascist crusader. Redgrave comes reeling across the screen like some flayed, tragic thoroughbred.

Redgrave in 1940 as an evacuee aged three: the photo appears in her documentary Sea Sorrow, being shown at Cannes.

But her acting career is only half the story – or rather it travels in tandem with her leftwing activism. Back in the early 70s, she threw in her lot with the Workers Revolutionary party and helped purchase an Edwardian mansion (nicknamed “the Red House”) for use as a Marxist college. Her most notorious moment came during her Oscar acceptance speech for Julia, when she lambasted the actions of “a small bunch of Zionist hoodlums”. Never mind that Redgrave was referring to members of the Jewish Defence League, who had burnt effigies of her in the street. Many assumed she was insulting the whole state of Israel and the resulting uproar had Hollywood running scared.

Four decades on, she remains a polarising figure. Redgrave’s detractors revile her as the archetypal champagne socialist, a rich and privileged actor come to lead the masses along the path of righteousness; tell them how to lead a more worthy life.

Redgrave bridles. She can’t think what I mean. “I haven’t ever told anyone that. Why would you say it? Why go there? All I’m saying in this film is to obey the law. I go to prison if I don’t obey the law. Shouldn’t the UK government obey the law too?”

OK, fair enough. But doesn’t activism sometimes put one at odds with the law? Has she never found herself in violation of laws that she believes are unjust?

“Of course,” she allows. “That’s a legitimate question. But that’s not what we’re talking about. We’re talking about human rights conventions, which are mandatory. And so we are saving lives when we obey the law.”

In the past she has described Jeremy Corbyn in utopian terms, as representing “an English spring”. Today she says she’ll be voting Labour in the June election. “But Jeremy Corbyn is not the issue. I think he’s a loyal party activist. He’s trying to be a loyal leader. But the issue is to go on record against Theresa May and what her government is doing and will continue to do. Democracy is at stake. That’s why I’m voting Labour.”

There was a point in the hospital when I thought that I didn’t want to live, that it was too hard to struggle Vanessa Redgrave

By this point, I find I’m quite enjoying our joust. Redgrave seems perpetually on a hair trigger, liable to erupt if she’s prodded. But her scolding strikes me as being entirely without malice, while those sudden rages can be curiously bracing. If nothing else, they prove that she’s still in the fight, bloodied but unbowed. Over the past decade she’s lost her younger sister Lynn, her younger brother Corin and – most shockingly – her eldest daughter Natasha, killed while skiing in 2009. For all that, here she is, “Big Van” as John Osborne used to call her, wired on coffee and tearing strips off reporters.

In April 2015 she suffered a heart attack, and it was touch and go for a while. Now she’s quit smoking; hauled herself back from the brink. “I do miss the cigarettes,” she says. “But I’m so glad I haven’t smoked, not even one puff, since I was in hospital. I stopped because I decided that I wanted to live. There was a point in the hospital when I thought that I didn’t, that it was too hard to struggle. Then I decided that no, I had more energy than I thought.”

Life, she explains, has felt more precious since then. She adjusts her position, stares off to the side. A moment ago she was full of imperial fury – like Lewis Carroll’s Queen of Hearts, demanding heads on a plate. Now, all at once, she has turned contemplative and sad.

“Yes,” she says. “You bet I feel it, the preciousness. I mean, I did realise it before, because you know that my daughter was killed in a skiing accident, and so that had already brought it home. And then my sister battled cancer for a long time and she taught me that too, because her life was so precious. So yes, death does remind you that life is both finite and infinitely precious. I mean, it’s one thing to just reel that off. But it’s another to realise that this life is important. I’m talking about our collective lives; the ones we’re all leading. Just being alive, staying human, I think that’s infinitely precious.”

Since we’re talking about life, I figure I can invite her to look back at her own. Any major regrets? Anything she might change? Would she say that hers has been a life lived well? But this is too much to bear; she can’t get over my gumption. Her cackle is almost enough to rattle the coffee cups on the table.

“Oh now, come on,” she explodes. “How can I possibly say? There you go again, trying to bring it back to the personal.” She takes a moment to collect herself. “I don’t think my life has been badly lived – but I don’t think of it that way. And nor should I. I think it would be awful! It would be like I’m speaking at my own funeral, lots of eulogies and stuff. “No!” she says. “No! I’m not prepared to do that, thank you very much.”