Chuck...A Brit?

Just watched the OG retrospective, and although I already knew Ed Westwick, was British, it was still somewhat shocking to hear Chuck speak with an English accent. For some reason, I am always impressed by English actors who can pull off an American accent. Some can only do it for their scenes; others, like High Laurie, can turn it off and on, like a switch, which made for some funny House outtakes. Does anyone know if Ed or other English actors who can do the American accent at will?