For reasonably small amounts of money, I've been using my Bank of America card at a Barclay's ATM. There's an agreement in place between the two banks such that you don't have to pay a foreign transaction charge, atm fee, etc. Basically, it's a straight across conversion using the forex rate. You simply can't get a better rate than that. If you need to pull out larger amounts of money, you can ask BofA to increase your withdrawal limit to $1700 a day for a week or so. It may be a bit of a pain in the ass if you're pulling out money for a mortgage here, but it's great for everything else.
If you don't already have a BofA account, you can open one online. You may need to temporarily provide a US address, but I was able to update my records to the UK, once I arrived (many other banks wouldn't let me - capital one, ing direct, etc.) I can't really recommend BofA on their own merits, but this partnership agreement makes them pretty desirable.