Heh, I once encountered a guy online who insisted that Keira used to work as a check-out girl at some grocery store near London, and his big problem with her was that "she doesn't REALLY talk that way!!!"....
Just a pointless anecdote for ya.
Anyway, Hazzle -- I maintain that her American accent in The Jacket was, in and of itself, convincing. I think that anyone seeing the film who didn't know in advance that Keira was English would not have thought twice about her accent. The crux of our disagreement on this matter seems to be that she had trouble doing BOTH and accent WHILE giving a good performance. Something was off there.
And yes -- I have dealt with alcoholism, and I found her portrayal of an alcoholic to be rather derivative. I didn't even really get the sense that she was a drunkard until reading later that's what she was supposed to be. To me, she just seemed depressed, in a general way.
Although I bet that she could pull the part off much better now than she could back then.