SpookyFrank
A cheap source of teeth for aquarium gravel
What they taught us at drama school is that with few exceptions, US accents have very little "tune". British accents use a constantly changing range of pitches and inflections to communicate non-verbal meaning (emphasis, attitude, subtext). Outside of the southern and south eastern states, there's much less tune. Emphasis is done with a combination of pace, pause and volume.
British actors certainly seem able to play American more easily than vice versa, I suppose it's easier to remove some of the expressiveness from your voice than to add more.