Hey there, welcome to the boards!
First of all, the GBP is doing so much better against the dollar that I don't think you'll be losing much money with the % fee that visa/mc charges. Seriously. But why can't you use the ATM's? We can use the atm's in europe, I can't imagine why european cards wouldn't work here. Hmm. But if you're really concerned about it, go to a bank and do your exchanges there. The US has tons of banks everywhere, at least as many as the UK, if not more. Good luck and have fun! If you're planning to go to Vancouver BC stop by Seattle!