If you're looking for hardcore, you can never go wrong with Dr. Freeclouds in So-Cal. I was turned on to the place after I met Ron D Core at a party, and they have the best hardcore selection I've ever seen, hands down. Hell, any place that stocks things by Cunt Records gets my seal of approval. That said, they do tend to focus more on the American Hardcore and Gabber side of things, I don't recall there being an insane amount of UK stuff (But this was two+ years ago as well, so who knows?)
Plus, since they're in So Cal, even if you ordered online I'm sure the shipping charges would be nothing like they are from the UK.
http://drfreeclouds.com/