The reasons for this are many fold, ranging from poorer opportunities and education to more severe punishments for crimes committed.
That said, extrajudicial killings are extraordinarily rare in the UK. I don't think anyone, even someone from those communities, would ever expect to die during an interaction with the police in normal circumstances. While in the US it's becoming clear that there is a real fear of any police interaction being escalated to that point.
It does happen though, and it disproportionately happens to black people. https://twitter.com/korrinesky/status/1267085008210210816?s=...
Moving away from law enforcement to mental health treatment we see black people are more likely to be detained under the mental health act, spend longer in detention, are more likely to be on CTOs, are more likely to be on forensic sections. In hospital they're more likely to be subjected to seclusion, restraint, and rapid tranquillisation.
UK police are racist and they subject black people to disproportionate use of police powers.