I agree that racism can be shown, and 12 Years a Slave was passed with no cuts, though caveated for strong violence, injury detail, sex, nudity and racist terms
Other films have cuts made for extreme racism: BBFC’s statement when explaining the certification for Love + Hate: “Works which actively encourage and endorse racist attitudes and behaviour have been placed at the adult category, cut or rejected in the past”. but, as in this case, they can stand when the film as a whole shows adequate mitigation.
I think what counts as “extreme” might be the crux here.
And of course a BBFC approved film, which contains adequate mitigation (both in story arc and in cuts to the most excessive passages) does not compare to quick-fire live sets