Any non-perishable food items you'll REALLY miss in the first few months (they don't really have ranch dressing here other than Newman's own).
Check and see if your favourite hair products, face and body products and make up are available here. I had to find a lot of new products when I first moved. I still haven't found replacements for some things.

US measuring cups (they are larger here) if you have a lot of US recipes you'll use.
And that's all I can think of. I really miss my favourite Murad moisturiser, I wish it wouldn't be so expensive to get here.
Oh, and if you like Clinique lip balms, the lovely coloured ones in tubes, they don't have them in the UK anymore. My mom's sending me some from the US where they are still for sale as they are one of the best selling products.