I've always notified my American Bank, prior to coming to the UK, that I would be in this Country....and I used my Debit and Credit Cards with no problems anywhere. My husband did the same with his England Bank, before traveling to the U.S., and again, no problems.
And yes, I would definately have some cash on arrival....just pocket money for a few days. I "think" someone told me that the Post Office does not charge a fee for issuing foreign currencies, but not sure about that one.