As an example, Barclays charge 2.99% on overseas cash withdrawals plus a flat fee of £1.50 if you use an ATM not within the Global Alliance group. (The ATM operator might charge an additional fee but, if so, that should be indicated at the ATM).
So withdrawing the equivalent of £100 would cost you £2.99 + £1.50 = £4.49 at most Spanish ATMs. (There's no Spanish bank within the Global Alliance Group although you might find a few ATMs operated by non-Spanish Global Alliance members, such as Barclays, Deutsche Bank or BNP Paribas).
However that's still probably the cheapest way of getting Euros because the exchange rate charged is the commercial inter-bank rate, rather than the far worse rates available at UK (or Spanish) exchange facilities (such as Travelex or the Post Office).
Chris