In general, Marks & Spencer and the Post Office usually top the list of recommendations from AB contributors. However (even if your bank charges you for using your card abroad), it's still better to use a debit card at overseas ATMs. (I never take more than a few euros with me - just enough to keep me going until I can find an ATM. It's safer than carrying large amounts of foreign currency and achieves a better deal overall).
Try using TRAVELEX online. They have outlets at a number of airports in the UK and provided you order early you generally get a better rate. If you pay by Debit Card, no charge.