To my knowledge there isn't a way to do this on any site. I think the reason is that there isn't that much of a difference between British and American English. Sure it's not the same (different pronunciation, a few different words), but we still understand each other. From what I understood, there is a bigger difference between Portuguese in Portugal and Brazilian Portuguese. I could be wrong, but that seems like the reason. So as far as I know, no one dubs American movies with a British accent.