I always admired actor Tim Roth’s British accents, which as far as I could tell were absolutely pitch-perfect. What I didn’t realize was that, um, he’s British. So that’s why, you know, he can do the accent…
What he *does* have is an impeccable American accent, and does excellent regional British accents as well.
Another British actor with a great American accent is Hugh Laurie (A Bit of Fry & Laurie, House).
Aussie actress Toni Collette also does a good American accent, although perhaps not in a class with Roth and Laurie.
And speaking of Toni Collette, I quite enjoyed the movie Japanese Story.
Not that you cared.