Louise, get a Halifax Clarity Credit Card, this is the best one available for overseas use with no charges/commission when purchases are made (tavernas and shops) not to be used for cash from ATMs though as this will incur charges. You don't have to have a Halifax account to get one.
At this moment in time you'll probably get an exchange rate of about 1.24, maybe 1.25, far better than the exchange rates in the UK, even online exchange rates. Take some euros with you as well.
Most of the tavernas I know accept credit cards, the exceptions that I know of are Fantasea and Beachcomber.