Nationwide seems to be the best credit card because it doesn't load a currency exchange charge. However, there is an interest charge on cash withdrawals on the card even if you pay it off in full and it has a 1.25% cash withdrawal charge. If you go for their Cash Reward card you get 0.5% cash back on all purchases. Don't get the card too far in advance of your trip because the cashback is 1% for the first 6 months.

You also need to look for a small charge made by virtually all the US banks for each withdrawal. You can save money by making 1 large withdrawal rather than a few smaller ones.

Other cards offering no exchange charge are Lombard, Liverpool Victoria and Saga. The latter 2 only waive the exchange rate charge in Europe so should be avoided for the US. You have to be over 50 to get a Saga card. Lombard's interest rate and fees are higher that Nationwide's.

The best option of all is the Nationwide debit card. Obviously you have to bank with them to have one of these but the charges on the credit card don't apply.