I think you yourself, AOG, have made the point that Black people in the UK have been disproportionately the subject of "Stop and Search" measures. It's a long way from stopping black people more often to gunning them down with impunity -- and anyway there are clear differences in police practice between the US and UK beyond habitual gun use -- but in that context it's not necessarily unreasonable. If police had an extra measure, killing suspects in certain situations, then (a) more people would be shot dead anyway, and (b) it's plausible to suggest that black people would be disproportionately the victims in such cases.
As it happens I don't think that this will be nearly so bad a problem in the UK, as compared with the US. Aside from the relatively non-existent culture surrounding gun ownership here, so that the police would have less reason to suspect that such-and-such a person is carrying a gun and therefore an imminent threat, we also have much more robust and independent investigative measures against police who may have overstepped the line. The IPCC is called in automatically whenever a person dies in an incident related to police action; in the USA an such investigations are often internal, so that it's the friends and colleagues doing the investigating (conflict of interest right there), while any indictments that could be prosecuted end up before a Grand Jury (citizens, who tend to want to trust the Police), as opposed to the CPS (independent body).
Or, in short, we have much better safety nets to monitor the Police than the USA do.