Don't panic if you have ATM trouble at the airport, as both the tube and express trains accept credit cards for payment. So you can skip the airport ATMs entirely (as I did) since you will have another shot at hundreds of ATMs when you reach central London.

I would hasten to add that most London banks (unlike US banks) do not gouge ATM users with a surcharge. So if your own bank doesn't tack on a charge, you are getting a very low-cost exchange rate.

In my case I used one of the Citibank branch ATMs which are conveniently located near the business district (Monument) shopping (Oxford Circus) the theatres/tourist sites (Charing Cross) or Canary Wharf.

I understand that Bank of America also has an agreement with a large London bank where it will waive ATM surcharges.