Essentials for me would definitely be things to make Mexican food with--mainly chile verde sauce in a can, for easy green chile enchiladas and burritos. I have yet to find that, all I can find is Old El Paso in Morrison's, and it tastes *alright* but not quite the same
Thankfully I haven't had problems finding tampons/pads here in the UK; I use pads generally (it's what I was raised on, and it's the most comfortable for me IMHO) and Tesco has the name brand stuff I use here in the US (without divulging too much info, I stick with the brands I know--my body doesn't really like new stuff
). And I actually don't mind English peanut butter, even though I've heard horror stories
So yeah, stuff for Mexican food if that's your deal, maybe stock up on some clothes (I find I have cheaper and more varied options here in the US). Definitely OTC stuff like Tylenol (or PM) and stuff--nothing makes me sadder than only being able to buy one tiny box of paracetamol and only tiny box of ibuprofen, like I'm gonna overdose on the stuff and need to be protected