Paying with UK Credit Cards

Most cards charge 3%. Always Always pay in €€€ never £££. The rate you get from the Spanish card handler is shocking. If you pay in €€€ you will get the bank to bank exchange rate
Lets say the current rate is 1.12 when they offer you the £££ option it's usually at 5 cents less. So even if you are paying the 3% you end up better off

All credit/debit cards are normally accepted worldwide as long as it displays the VISA logo, although I'm pretty sure there can't be many cards that does not have this nowadays.