I've lived in London for over 15 years now and, although my accent was never terribly strong, it has softened considerably over the years. Nonetheless I have not acquired a south London accent and it is fairly clear to most Brits, if not most Americans, that I am from North America. It is not uncommon for me to get the "are you Canadian?" question when meeting new people. I always thought it was because people were uncertain as to whether I am a Yank or a Canuck and, assuming that a Yank would less likely to be insulted by being mistaken for a Canuck than the reverse, that they were simply erring on the side of caution. I am no longer certain that that is the case.