None of the major banks or building society charge fees for ATM use over here. The exchange rate will be applied by your credit union, not the bank you withdraw the money from If they do charge a fee, they generally have to tell you (and also tend to be found in pubs, some shops, motorway service stations) and give you the option to not get. Watch out for Travelex branded ones, as they look like they shouldn't charge a fee, but they do However, at LHR, it's likely the ones next to forex places are bank operated, rather than Travelex operated, and so should still be free! Basically, you are more likely to encounter free ATMs than ones where you have to pay a fee