I don't really see much of a point in re-recording things just to have a different accent spoken by the majority of the characters. I think primarily who records the VA depends on what studios pick up the rights quickly enough to do it, though I imagine the profit margin of redistributing anime is probably so slim that only a handful of companies would want to do it. So distributors in the UK likely don't want to add any overhead cost to their product if an English dub already exists, even if it's American English.

Rereading that I see that you also mentioned movies... not sure. I could see it happening with films tbh, though likely the other way around; a film already dubbed with English actors being re-dubbed with Americans, just because of how a lot of Americans would perceived a dubbed movie if it wasn't done with American actors.