I'm not sure that it's a group effort to replace African American actors. Spike Lee himself was repeating the "British actors are better trained" trope. He got the platform he said it on to scrub it from the net, but I've posted the audio here a few times. Spike is straight AA, and personally set off the renaissance of AA films in the 80s. I say all that to say that this perception was/is apparently widely held if a Morehouse man was saying it publicly ears ago.
The Morehouse man who wasn't ever going for it, Samuel L. Jackson, tried to call out the past Hollywood decisions that created the opening for Black Brits.Hollywood casting rappers in big studio films,and dramatic roles instead of young Black actors trying to pay their dues. AA rappers with NO acting experience or training, getting nods over trained AA actors trying to get acting credits. I'm sure some capable and talented AA actors got discouraged and went back home after losing roles to Method Man.
First film I ever David Oyelowo in was Red Tails, featuring Method Man and NEYO in dramatic roles. Can't make this stuff up.
Only one person in this picture has any kind of theater training
Progress of that next pool of AA actors was blocked because of casting decisions to put marketable AA rappers/singers in roles in film & tv. A lot of the talented ones said fukk it and left the business. Equally well trained Brits came in. Hollywood found a loophole to get well trained hungry actors to play roles for less than what an equally trained American actor would get. Brits did well in the roles they got, the rest is history.