ATMs are poor VFM IME.
I think there's a transaction charge as well as commission (charged by your own bank )
I think this adds up to almost 5% with my bank.
Convenient yes, cheap not.
As others have said, it depends on the bank and on the ATM
I use a Nationwide debit card (no charges, as adamski said), and if you avoid the ATMs that charge, and also avoid the ones that do the exchange for you so that you withdraw a stirling-equivalent, the exchange rate is good and you don't get charges
I use ATMs to get cash all over the world - the only time I use a currency exchange is if I am going somewhere where there may not be an ATM