Who is your bank in the UK? I know that Barclays is a "sister" bank with Bank of America, so if you use your debit card to withdraw money from a Bank of America ATM, and you won't incur any charges. I do this and have found that the exchange rate is as competitive, if not better, than you would get purchasing $$$ at any Bureau de Change in the UK.
If your UK bank is not Barclays, go to their website and see if they have a relationship with any US banks.