Not sure what healthy U.S. foods could be stocked here in the U.K. Like Britwife said, it's tough to stock fresh U.S. produce.
However, I too am very disappointed with the crap that is here for expats. Yuck. Frequently, I log onto FoodFerry or SkyCo only to abandon my cart at the end of shopping b/c I realize I really don't want or need to eat that food.
I wish I could get VitaSoy soy milk (hate hate the soya stuff here - blech), decent whole coffee beans, California Mexican food, and well, a Whole Foods Market would suit me fine.
At least SkyCo and the like could stock the VitaSoy though! Maybe some TigersMilk bars, too.