Not only that, but a lot of British films will make british-only casting decisions, eg the Harry Potter series. I've never heard of a major American film making that choice, and it would probably be blasted as xenophobic if it did.
Er how many American movies have a single non-American? The only foreigners that consistently find employment in American cinema are Britishers, and that’s because the British English accent is perceived to have an at least equal prestige to the American one. You’re not going to find many main characters that are, say, Middle Eastern or South Asian or African.
