I'm American and (unfortunately) never been to GB, but there are a few things I've heard you guys talking about that I know we don't have. HP Sauce and Marmite. I have no idea what either of those things are. Plus, we have different candy. We have "some" Cadbury chocolate, but not much, and what we do have (Except for the Creme Eggs at Easter time) has to be bought in a specialty store such as World Market, etc. (At least, it does where I live--Atlanta, GA.)
We also don't have the same brands of tea that you guys do. Our major ones are Lipton, Luzianne, Tetley and Bigelow. Maybe you do have some of those brands...not sure.
All I know is that here in America, I can't find clotted cream, so have your friend bring some with him/her and drop it off to me!! :)
Oh, and some of the biscuits/cookies that you guys eat can only be found here in specialty stores. McVitties...is that right? Things of that nature.