I think the American flag is more a marketing gimic than anything else.
I've tried a few of these so-called American foods at both Tescos and Sainsburys, and found they haven't been anything to write home about. However, Tescos has some American saltines that are probably the closest you'll come to proper crackers.
As far as the doughnuts are concerned, in the five years I've been here, I've had one good doughnut. I found it in, of all places, a gas station in Chiswick (London). For some reason, they were selling Mr Doughnut's brand. My hubby says that there used to be lots of Mr Doughnuts around, but that's the only one I've ever seen.