The only charge for ATMs usually comes from the US bank, not the UK bank. Most of the ATMs operated by UK banks note on the screen that they do not charge for the transaction but that your own bank may charge you. However, be aware that ATMs that are not operated by a UK bank (like freestanding ATMs in pubs or convenience stores) usually charge a fee. My US bank does not charge me a fee for using a non-network ATM, nor do they charge me for a currency exchange, and if I use a UK bank ATM, I don't get stuck with a fee from that end either. ^ (I'm one of the few Americans that actually likes my bank)

I stay in the UK for 3 or 4 days at a time and usually only withdraw 50-60GBP once from an ATM that I use for things like cover charges, cocktails, etc. I do everything else (including tube tickets, shopping, and fast food) with a credit card. Credit cards (AmEx, MC and Visa) are widely accepted but Discover card is practically unknown.