Hmmmm while some Americans CAN do a decent English accent, what many American s obviously think is an English accent isn't. Dick Van Dyke in Mary poppins etc Also even Daphne in Fraser who is actually English PUTS ON her accent for that to sound like what Americans think is an Enlgish accent.
I do know that one the latest film, there were a lot of non-English people working on the production here in England. (2 of my friends (who don't know each other)) worked on it, and said it was pretty varied set of nationalities.