There are ATMs practically everywhere in Europe and the UK including a cashpoint in the Nido lobby. Most students find this to be the easiest way to get cash. However, before you leave, speak to your bank about (1) the high fees they may charge for overseas use and (2) not flagging your account as fraudulent due to the overseas use.
To counter the large fees some students take out large amounts of cash at one time. If you choose to do that be careful and watch out for pickpockets!
Western & Northern Europe:
Used in most places.
Eastern & Southern Europe:
Many places do not accept credit cards. Those that do more widely accepted MasterCard and Visa over AMEX.
as of February 2013
$1.57 = 1 Pound (£)