What if the black protagonist of Spike Lee’s “BlacKkKlansman” was not a cop infiltrating a racist institution in order to bring it down, but a true believer finding a violent outlet for a psychological Molotov cocktail of internalized racism and pathological self-loathing? This is, roughly speaking, the extraordinary premise of British actor Adewale Akinnuoye-Agbaje’s directorial debut “Farming,” which is based with queasy authenticity on his own experiences as a teenage member of a 1980s white skinhead gang in Tilbury, Essex.

Unremittingly, bludgeoningly bleak in its portrayal of his own degradation and humiliation, and displaying only a passing interest in his eventual rehabilitation, the film is remarkable for its lack of self-pity, but it makes the experience of “Farming” a merciless one for the audience too. The process by which a young boy can be turned inside-out against his own skin is scary; scarier still is the sense that deprogramming this mindset, undoing this damage, and holding the guilty to account is the work of at least a lifetime, and it may never be complete.

The term “farming” was the peculiar euphemism given to the practice, prevalent in 1960s and ’70s England, whereby working or studying Nigerian parents would pay white British families to foster their children. And so the film opens with Femi (Akinnuoye-Agbaje playing his own father) and Tolu (Genevieve Nnaji) tearfully handing over their infant son Enitan and a wad of banknotes to Ingrid Carpenter (Kate Beckinsale), a working-class wife from a Gypsy background, living in a terraced house in Tilbury.

The setting — the exact house in which Akinnuoye-Agbaje grew up — convinces in a way that Beckinsale cannot, even with her fine-boned beauty toned down in a frumpy housecoat, and a broad cockney accent flattening her vowels. But that might also be down to the irresolute portrayal of Ingrid, sometimes a monster, sometimes a Fagin-like figure, and sometimes an admirably steel-willed, working-class woman, struggling her way out of poverty while trying to do what she believes is best for the kids. Eni’s parents’ are similarly fuzzily drawn, their lack of contact with him despite their living nearby never satisfyingly explained.

As a boy, Enitan (Zephan Amissah) is artistic but shy, preferring to play behind the sofa rather than with his siblings (whom his parents also “farm”) and foster-siblings; at times Ingrid has as many as 10 black children in her erratic care. Suffering constant low-level racism at home, with Ingrid repeatedly threatening to send him back to “Wooga-Wooga Land” if he misbehaves, and the object of ceaseless bullying at school, Eni learns to hate his blackness. In one heartbreakingly horrible scene, he tries to scrub it off his skin before covering himself in whitening talcum powder paste, the ghostly, freakish result only occasioning more derision from his peers.

As a teenager, Eni (now played with bruised, brooding intensity by Damson Idris), still friendless except for the kindly ministrations of saintlike teacher Ms. Dapo (Gugu Mbatha-Raw), becomes the target of more overt violence, specifically falling afoul of the self-dubbed “Tilbury Skins,” a skinhead gang led by Levi (John Dalgleish). In the most literal example of the “if you can’t beat them, join them” mentality, he goes from Levi’s victim to his barely tolerated “pet” in the profoundly misbegotten belief that if he can outmatch them in the viciousness of his racial hatred, he will have found a tribe to which he can belong.

With a more dexterous screenplay, Akinnuoye-Agbaje would have been uniquely placed not only to deliver acute insights into the self-loathing on which the whole right-wing, skinhead mentality is based, and the skewed, animalistic logic of the bullied boy who becomes a bully in order to survive, but also a stinging indictment of the systemic racism that persists today in Brexit-era Britain. But while it never stints on depicting the mental and physical horrors inflicted on and by Eni, including several scenes of borderline unwatchable psychological torture, “Farming” falls oddly shy of pointing the finger of blame anywhere except at Levi’s snarling, repulsive villain.

There are searching questions never asked about the whole wretched system, whereby separating children from their immigrant parents was seen as a viable, even a charitable act. The underlying colonialist, “assimilationist” bigotry there — the idea that even being one of a dozen neglected and abused children in a poverty-stricken white household would have been preferable to growing up within a black family — goes largely unexamined. Indeed, with Eni’s redemption confined to a hasty montage and some titles describing how he went on to earn his law degree (culminating in a photo of the real Akinnuoye-Agbaje accepting his honors from a member of the Royal Family), the real hurt done to him and to thousands of other black British children, feels distressingly unresolved. If “it takes a village to raise a child,” as his grandfather says during Eni’s abortive trip back to Nigeria, then surely to break one so comprehensively must take a whole, complicit society.