Hmmm, never thought of "doughnut" as an exclusively British word. We eat 'em on this side of the Atlantic, too. (Our police are famous for their consumption of same.)

Perhaps the implication is that the simplified spelling, "donut," continues to gain acceptability here in the US, while the more traditional form is surviving in the UK.

That change in spelling is relatively new and still ongoing. When I was a kid, the long-form "doughnut" was still in very common usage, while "donut" was considered semi-ignorant, appropriate only for lowbrow advertising. (Very much like how the forms "lite" and "nite" are viewed today.)