The UK BAME population is largely located in a couple of Large more densely populated cities. Whereas the country as a whole is around 87% white, London has areas with < 50% white people, for example Newham/Straford is 70% non white[1].

Surely the geographical relationship between dense cities and their inhabitants demographics is more than enough to explain it.

[1] https://www.newham.info/population/