Aftersun, the devastating drama about a father/daughter holiday to Turkey in the late 1990s, has swept the board at the British Independent Film Awards (Bifas) in London.

Charlotte Wells’ debut feature, which has captivated critics and left audiences sobbing since its premiere at Cannes in May, took seven awards at the ceremony, which honours films made outside the studio system.

It won best film, best director, best debut director and best screenplay for Wells, 35, as well as prizes for editing, cinematography and music supervision.

Its stars, Paul Mescal and 11-year-old newcomer Frankie Corio, went home empty-handed; instead, best lead and best supporting performance went to Rosy McEwen and Kerrie Hayes, the stars of Blue Jean, another drama by a first-time film-maker.

Georgia Oakley’s debut, which premiered at Venice, is the story of a gay PE teacher battling prejudice in 1980s Britain.

This is the first year the Bifas have adopted gender neutral categories; this year they also introduced a best joint lead performance prize, which went to Tamara Lawrance and Letitia Wright as the real-life siblings who communicated only with each other in The Silent Twins.

Best ensemble went to Our River…Our Sky for a cast who played neighbours in Baghdad during the sectarian violence in 2006. Meanwhile, best breakthrough performance was given to Safia Oakley-Green for The Origin, Andrew Cumming’s thriller about a nomadic tribe that comes under threat from an ancient force after dark.

Kathryn Ferguson’s study of the activism of Sinéad O’Connor, Nothing Compares, won best documentary and best documentary director. Best international independent film went to Joachim Trier’s The Worst Person in the World.

This is the 25th year of the Bifas, and the ceremony was held in Old Billingsgate, east London, hosted by Ben Bailey Smith. The Richard Harris award for outstanding contribution by an actor to British film was presented to Samantha Morton. In her speech, Morton called for a general election, paid tribute to her Code 46 director Michael Winterbottom as well as behind-the-scenes crew, as well as addressing industry classism.

She finished by describing Lynne Ramsay, with whom she worked on Morvern Callar, as “a remarkable human being and the best film-maker — sorry to everyone here tonight, but she’s the best.”

Open Door, which helps young people without financial support through drama school, took the special jury prize.