We had no problems with either credit cards or using the ATM when we needed money. Just make sure you have pounds or euros for things such as cabs and some meals. It is not cheap in London, so be prepared

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One tip on credit cards and ATM's. I checked with my bank (Bank of America) and found that they were in a group with a number of European banks, one being Barclay's in London. I was able to use my ATM at those without paying extra. Also, check to see what the add-on for foreign purchases with your CC is. I think they all add 1%, but a number add an additional 2%. . I was able to find a couple of cards that didn't do the extra adding.