‘It’s so odd,” said the 86-year-old Muriel Box in 1991, the year she died, “why they can’t have the grace to say: ‘We know we have all the good opportunities as men to direct and do everything.’” In women, she said, the film industry “hasn’t any confidence at all … They never say to women: Let us try and see what she’s done.”

Box knew of what she spoke. Fifty-four years since her final film, she remains Britain’s most prolific female director. In the 1950s, she strived and scrimped, surmounted institutional prejudice and ill-health to make her 13 features. Now, they are being seriously re-examined, with major retrospectives at this year’s San Sebastián and Lumière festivals. Audiences are finding Box isn’t just worthy of interest because of her outrider status, but because of her quest to find adventurous new expressions of women’s lives.

Beyond the baseline of independently minded female characters, Box singled out emerging areas for women workers, such as the police (in 1953’s Street Corner), and touched on taboos such as child prostitution and venereal disease (1960’s Too Young to Love). Even a crowdpleaser such as 1957’s The Passionate Stranger, about a strapping but dim Italian chauffeur who mistakes the romantic fiction of his novelist boss for wish fulfilment, couches its feminism in a bluff and surprising way. As the hired hand reads his boss’s work – in which a pent-up concert pianist romances her driver – the action shifts from black and white into gaudy colour. Written by Box and her producer husband Sydney, this tentative postmodernism feels like the couple having fun satirising how domestic tensions filter into fiction. But never mind the would-be Latin lover’s assumptions about women’s fiction. Writing simply serves a straightforward purpose – economic self-sufficiency and enjoyment – for Margaret Leighton’s novelist, who gives her suitor the brushoff.

The Passionate Stranger makes the creative professions look more of a lark than Box found them. She was already 47 when she directed her first solo feature, the Passport to Pimlico-esque comedy The Happy Family. Her entry into the film industry in the 1930s ran in parallel to a highly successful career as a playwright; written with Sydney, her works – many of which featured all-female casts – became the most performed in Britain during that decade. Meanwhile, she hauled her way into film-making as typist, script-continuity checker, secretary to Michael Powell and then scriptwriter.

Despite winning a screenplay Oscar for the 1946 melodrama The Seventh Veil, Box struggled to make the next step to director. The couple’s projects were repeatedly turned down, but Sydney persuaded Gainsborough Studios to let his wife reshoot portions of Cockpit, a Richard Attenborough wartime vehicle eventually released as The Lost People.

The pair formed their own company: London Independent Producers. First up was The Happy Family, but despite business arrangements designed to afford her more control, Box came up against the same old attitudes. The received wisdom was that innate “sensitivity” made women better editors than directors. On The Happy Family, Sydney told casting directors that he and Box would co-direct, then let his wife call the shots.

Sometimes initiating projects through their company, sometimes working for the big British outfits, Box had to decide how feasibly she could challenge the existing framework of the industry – and wider society around it. Her sister-in-law Betty Box (AKA Betty Box Office) became a powerhouse producer, married to future Carry On honcho Peter Rogers – but Box never fully gave herself over to prevailing commercial mores. Even the relatively benign The Happy Family, with its bickering cockney family defending their house from demolition, had an unusually female slant. Her last film, 1964’s Rattle of a Simple Man – featuring Steptoe and Son’s Harry H Corbett as a Mancunian bachelor on a lad’s away day in Soho who scores himself an improbable evening with Diane Cilento’s buxom escort girl – seems to cede to the nudge-wink Carry On vogue. But it mocks male priapism and conducts an almost anguished examination of power relations between the sexes.

Muriel Box with husband Sydney Box and his sister, producer Betty Box. Photograph: Alamy Stock Photo

Making room for outsider perspectives sometimes meant stretching existing forms, as in The Passionate Stranger. Sometimes Box just delights in experimentation for its own sake. The riotous Simon & Laura, from 1954 – about a quarrelsome Olivier/Leigh-esque pair of married actors forced to put on a show of harmony for their new show – anticipates reality TV. It’s when Box is handling her most explicitly feminist themes that her artistic seams can show. Just as such debates were unresolved in 1950s Britain, her films occasionally have the feel of an argument being worked through in real time. Too Young to Love, in which a juvenile-court judge attempts to fathom why a 15-year-old girl would begin taking money and gifts for sex, becomes tortuously snagged on the generational conflict between mother and child. For the teenager, slipping into prostitution is a form of self-liberation from an oppressive household; the mother can’t accept her rejection of the haven she has slaved to create.

It’s interesting that in both Too Young to Love and Rattle of a Simple Man, Box equates female autonomy to some extent with prostitution, as if society at the time deemed both beyond the pale. But the two characters have self-assuredness and moral courage. There’s similar conviction in how frank Box’s films often are about sex, even when they’re old-fashioned in other ways. Too Young to Love makes a bald plot point out of abortion, still illegal when the film was released. Cilento, in Rattle of a Simple Man, looks like a swinging 60s glamourpuss, but her guilt-free attitude to sex is less self-consciously saucy than an acknowledgement that women should dictate and enjoy their sex lives. It’s not clear how much Box did, but – with her first boyfriend a colleague of her father’s more than a decade older than her – she seems at least to have a firm disregard for convention.

Perhaps that self-assertion is what allowed her to pull through when Sydney, despite his facilitating role in her career, didn’t turn out to be such an ally. In 1964, an intercepted phone call revealed he had been keeping a mistress in another London house. He later admitted he had been serially unfaithful during their 30 years of marriage.

They divorced in 1969; with her works struggling at the box office, her directorial career didn’t survive the wreckage. She later founded Britain’s first feminist publisher, Femina Press, and her parliamentary campaigning led to meeting and eventually marrying the lord chancellor, Gerald Gardiner. Her films fell into the abyss of the unwatched until her name resurfaced in the 90s.

There’s a note of disquiet in critic Elisa McCausland’s essay for the San Sebastián retrospective, about the need to periodically remind that Box and her fellow pioneers existed: “We might wonder if these cyclical recovery strategies are not a sign of essential failure. If we have to unearth the same names every couple of decades, instead of definitively including them in the history of cinema, it is because something about them does not appeal beyond political and cultural cosmetic operation.” Hopefully, more opportunity to see Box’s brisk sketchwork of the battle of the sexes on the big screen will settle that debate.