As per your suggestions I went to Selfridges at the Trafford Centre in Manchester today. I came back with macaroni and cheese, maple syrup and peanut butter (life's little essentials). I was in heaven.
Also here's American foods i've found at general grocery stores:
Asda - Skinny Cows, Pepperidge Farms cheesy fish snacks, Betty Crocker mixes, Paul Newman dressings etc...
Morrisons - Miracle Whip, Grape Nuts, Betty Crocker Mixes, Paul Newman dressings etc...
Tesco- Good REAL American Bar BQ sauce (can't remember the brand), Betty Crocker, Pepperidge Farms Choc'chip cookies, Grape Nuts, Paul Newman dressings, Jerky, some little coffee packets from San Fran, Skinny Cows.
If anyone finds other American goodies please let me know.
Happy hunting.