Because many of the British actors we see were trained in the theater, where they would get to play all sorts of character roles, they may bring more versatility to the screen than their American counterparts. Many American actors on movies/TV have no theater experience, so wind up bucketed into similar sorts of parts on screen based on their particular physical attributes. So it can feel that British actors are superior to American actors.

But a British acquaintance once pointed out that there are "bad" British actors who we are blind to, because the accent is all the acting they do. That as an American, I may not hear or see that the acting is bad, but as a British person, it was painfully obvious to him. Food for thought.