Bump, to clear up an inaccurate criticism...
Just read that the girl who is doing this was born in Berkshire, so she's English. Criticise her accent if you like, but it's authentic British. :p She does have some twangs which I don't know if they are Ascot twangs or her picking up some parts of the American accent, but she still says a lot of words in a very British way. Going by interviews, not the game.
EDIT: Listening to other Berkshire accents on YouTube, I'd say she hasn't picked up all that much of American, they sound very similar.