I agree completely about not bringing too many American things and learning what you like among British things instead. However, I do like Hidden Valley Ranch dressing and Cheetos. There's an American store near me that sells both, but they are outrageously priced. Also, I bring back big sticks of deodorant (the kind I use is available here, but it's tiny and runs out too fast), big boxes of Q-tips, and NyQuil. Because as comforting as Lemsip is, sometimes you just want something that knocks you out, and I've never found NyQuil's equivalent in that regard.
Personally, I'm glad there are things you can't get in the UK. Different countries should be different, and if you want everything to be the same, stay in the US (not saying that's what the OP wants, just a general statement). The last time I went to visit my family I brought back some Nutter Butter cookies for my colleagues, and it was hilarious to watch their reactions. "Peanut butter sandwich cookies!" one of them exclaimed "How exotic!"