If in London, try Shephards Market on Drury Lane in Covent Garden (there is also one next to Richmond train station but the selection is more limited). Amongst other things, at times they have had Heinz Dill Pickles and Pickle Relish (the real stuff imported from US!), Cherrios, Lucky Charms, Apple Jacks (all very expensive), Libby canned pumpkin, Crisco pots, Snyders pretzels, Aunt Jemima mixes, bisquick, Jif PB, Oreos (from the US, which I am told are different to the ones sold in the UK), Hersheys, Reese's, etc. Occasionally they've had Welchs grape jelly and grape juice. I haven't been in for a few months but they generally have a good selection...a friend of mine and I go for the dill pickles, but sadly, last time there were none to be found.
The Canadian shop has a good selection, but some of the chocolate (Hersheys and Reese's) is the Canadian version and not the US version (for example, some of the Hershey's is made by Cadbury's so creamier dairy milk instead of milk chocolate).