The British pound is currently worth about 1.6 times the American dollar, although it varies from day to day. (£65 is thus about $104; £12 is $20)To get the best rates, usually it's best to charge everything you can on a credit card rather than convert money.
To be able to have some British pounds right from the start, get American Express Traveller's Checks issued in some British pounds rather than in dollars while in the United States - AAA can issue them, as well as American Express - so that you can have them changed easily right at the airport - the American Express Exchange Booth is best because you won't be charged another conversion fee.
Inside Britain, it is more difficult to find places to change American Express Traveller's Checks that are issued in American dollars, but many places will take those issued in British pounds.
Many of the major US ATM cards will also work at cash machines in Britain, so bring yours along.