My local Tesco (the massive one at Surrey Quays in London) now has a big American section in the World Foods aisle.
As of Saturday, it had: Apple Jacks, Lucky Charms, crisco, Karo syrup (light and dark), tins of egg nog, several types of Pop Tarts, Jif peanut butter, Stove top, Hamburger helper, A&W root beer (regular and diet), Butterfinger, Three Mustekeers, several types of Hersheys bars (though strangely not the original), and Libby's pinned pumpkin. That's just what I remember, anyway. Next time I'm there I'll take a photo.
(Notable exceptions: no goldfish crackers and no Kraft Mac & cheese)
The prices were good - better than Waitrose for the tinned pumpkin and the other items were WAAAAY cheaper than Cybercandy or imported online prices. I'm interested to see if the selection changes throughout the year.