I recommend using the Sony store with Entropay, rather than relying on gift cards.
You just go to
www.entropay.com and create a virtual Visa card and transfer money to it from your real card or direct from your bank account, similar to Paypal. Then create an account on the Sony store using a US address (any should work as long as the zip code is valid) and then when you add a credit card for payment you just enter the details of your virtual card from Entropay.
Quick, easy and superior to Waterstones until they get their act together and greatly increase the amount of titles they have available. Like most of you I'd be very happy to order from a British store if they actually had the books I'm interested in.