There are a few American food shops that have physical premises, but I think they tend to be quite small. I agree with Chary that you'd struggle to find business for a full-size supermarket that sold only American food. I do like to go to American Fizz in Bournemouth, but I only go once every few months, and the local British people really only buy chocolate and snack foods.
However: I agree with the ranch dressing. Big bottles of Hidden Valley, please! Cheetos. Root beer, Mountain Dew. American Fizz has Jarritos sodas, and I usually buy a few of those. Kraft mac and cheese. Canned pumpkin. Seasonal things like Peeps.