I'm not sure if this is new, but I don't recall it mentioned in the time I've been keeping up with this thread:
A GREAT find of American food in Milton Keynes shopping centre. It's a store called "Let's Eat" that mostly does hot food/coffees/café stuff, but they've got a little import food section. The selection is actually better than anything I've personally found in London (thought the prices are just as bad). Lucky Charms, Apple Jacks, A1 sauce, cake mixes, Crisco, lots of different chocolate bars (Baby Ruth!!), barbeque sauce, peanut butter, Fig Newtons, Nilla wafers, Oreos, Chips Ahoy, pancake mix, pancake syrup, black beans, etcetc. There's more, but I can't remember everything!! I've been to Selfridge's, Harrod's, and a couple other in'tl food stores in London, but this little place is the biggest selection of American foods in any one place I've seen so far. £6.75 for a box of cereal, but if you're desperate...:-P