jmgriffith

I'll reiterate what has already been said.
Definitely open up a Bank of America checking account and get a debit card. We did this earlier this year and dropped all of our vacation cash into the account. Withdrew cash from an ATM every couple of days. No international transaction fees, no ATM per-use fees, nothing.
If we found ourselves low on cash we used a Capital One credit card...again no fees.
Barclays ATMs are all over in London so I don't think you'll have any problems finding one. I printed off a Google Map showing all the locations though in case we needed it (never did, but you might want one for peace of mind).

Mimar

Perhaps the OP is traveling beyond London -- to a locale where Barclay banks are not so common. Though I don't know where that is. Northern Scotland?

AlessandraZoe

jmgriffith--Actually, we had rather bad luck with opening our Bank of America banking accounts. Had never had a problem with our BOF credit card here in the US or overseas in over two decades of use; however, we went insane trying to get our PIN numbers for the new bank accounts from BOF. Had they had an branch within 100 miles where we lived, there would have been an easy fix--we could have just walked in, showed our documentation, and it would have been solved. We ended up closing the accounts in sheer frustration.

I highly recommend Schwab online banking. Lovely to deal with, no foreign transaction fees and they will refund foreign ATM charges up to a certain amount within 45 days.