The OP doesn't tell us the currency of his card, though it seems it may have been a USD card. It may be that if it was a non-£ card, there's a currency exchange fee involved.

In any case, as NickB points out, bank ATMs in the UK don't charge fees, unlike ATMs I have used in the US for example. Whether the OP's bank charges an ATM fee and/or currency exchange fee is a separate matter.

The exception to the above comment is 'convenience' ATMs, the kind operated in 24-hour grocery stores and (I imagine) bars and clubs. AFAIK, there aren't any of those at LHR.