My personal experience in London with ATMs is amazingly bad. ATMs would routinely offer an outlandish exchange rate for withdrawals. Even using my trusty Scwab Bank card, I was offered 10-15% "fees" or exchange rate shenanigans that overrode my card's fee-free capabilities. Just used CC thereafter.
London is an exciting, dynamic city in which one must keep one's financial wits about oneself at all times.