David Lammy, the Labour politician and candidate for Tottenham, is standing in a Commonwealth war cemetery in Voi, southern Kenya. Unbeknown to many, certainly to me, this rural town near the Tanzanian border was a major military gathering point in the first world war. A war into which 2 million Africans were dragged, and 1 million died. In the beautifully tended burial ground, each British serviceman who fell in the surrounding area is commemorated with a single headstone, according to the egalitarian policy of the Commonwealth War Graves Commission (CGWC). Except the black Africans. The bodies of those British subjects who died for their country, then British East Africa, are not here. So where are they?

“You see the bush?” says caretaker Antonny Wachira Kimani, pointing to an overgrown wasteland beyond the perimeter fence. The two black men walk through the tall grasses and rubbish. “They worked together, but nobody take care of them,” Kimani observes with terrible matter-of-factness. “This is apartheid,” says Lammy, kneeling on the earth and clasping his hands together. “I want to find out how the hell this happened.”

So begins The Unremembered – Britain’s Forgotten War Heroes (Channel 4), a shocking documentary broadcast as we mark the centenary of the first Remembrance Sunday. Part detective story, part takedown of our selective and self-congratulatory culture of remembrance, this is the buried history of the establishment’s failure to acknowledge how the racism inherent in colonial rule continues to result in a whitewashing of the past. Here is proof – in the unmarked, unnamed burial of hundreds of thousands of black African bodies killed in the war – that the only way to take pride in our collective history is to acknowledge its shame. Say sorry, and mean it. Do something.

Unsurprisingly, this is not how this scandal plays out.

In London, delving into the CWGC archives with professor Michèle Barrett of Queen Mary University of London, Lammy finds out exactly how the hell this happened. Despite the CWGC’s pledge to treat everyone as equal in death – “What was done for one should be done for all” – exceptions were made. Barrett unearths a 1920s document by a Maj George Evans revealing that “everyone” meant white Europeans fighting on the western front. “Most of the natives who have died are of a semi-savage nature and do not attach any sentiment to marking the graves of their dead,” Evans observed. His conclusion? “I consider the erection of individual headstones would constitute a waste of public money.” A letter signing off on the plan was authored by the chairman of the then Imperial War Graves Commission, Winston Churchill.

A cascade of devastating moments follows. Lammy meets a man, Mwamkono Mwavaka, whose grandfather was one of the estimated 100,000 Carrier Corps – the men, women and children recruited to transport supplies to the frontline in what was effectively a death march – who died in the war. Only the hair shaved from his head was brought home. In a vast war cemetery in Dar es Salaam, Tanzania, where British and enemy German soldiers are buried side by side, campaigner Kathleen Bomani breaks down as she recalls going through every single gravestone and finding not one African name. “Where do I go in my own country?” she weeps.

Most heartbreaking is the urban wasteland off Pugu Road in Dar es Salaam, where Barrett reckons thousands of black African carriers are buried. No names, graves, nothing. The minutes of a CWGC meeting reveal a recommendation that the area “should be allowed to revert to nature as speedily as possible”.

Finally, Lammy returns to London and the swish headquarters of the CWGC for a reckoning with its director general, Victoria Wallace. It may be the most grim moment of this enraging documentary, which is saying something. Wallace responds with defensiveness, recalcitrance, ignorance (“I didn’t know that,” she says of the racial segregation at Voi. “But do we have the names? I don’t believe we do”) and a fallback on the increasingly trotted out argument that we cannot judge those racist times by the standards of our own. No apology, only a cursory mention of a £500,000 cemetery for black African Carrier Corps planned for Nairobi. The CWGC has since released a statement on its website, in which it argues the interview was “obfuscated”. “We have seen no documentary evidence that the claims about Voi cemetery are true,” it continues. “Indeed our archives make no reference to African war graves in the town.” One thing is for sure. This is not how the century-overdue work of making reparations looks.