We get ours from M&S using our store card (that way we dont have to pay untill after we come back) . M&S rate is one of the better rates and if you order more than £500 using your M&S card you get it delivered next day .
If you have a Nationwide credit or debit card it can be worth getting cash out of an ATM over there. I got $200 from the machine at the Boardwalk last week at an effective rate of 1.8855, including the cash advance charge.