Originally Posted by Calchas

Assuming it is on the Visa, Mastercard or American Express network, it should be accepted at any "cash point" (British name for an ATM) and indeed widely accepted in London for card transactions. However, you should tell your own bank that you are leaving the country, because I understand that US banks find it suspicious when cardholders use their card abroad and tend proactively to block the transaction.

London is a great city (I live here ) and I think it is an easy one for Americans to explore. I hope you enjoy your trip.