There are cards which do not charge for "overseas" transactions. The Post Office MasterCard is one, and you can use this card to buy currency online from the Post Office at no extra cost. The Post Office rate is usually reasonable, and you get four or more weeks credit at no cost.
Rover is correct, sorry Jimbo & Andy you're wrong. The "no commission" credit cards are by far the best value, even for "smallish purchases", much better than taking Sterling and exchanging it for euros (as Jimbo & Andy suggests), this can be one of the worst methods of exchanging currency as there is always a commission charge, usually 2%
As Rover says the Post Office credit card is good, as is Halifax Clarity, the older type Nationwide and Santander Zero and Saga credit cards.
I take three no commission credit cards and use them as often as I can (for purchases and definitely not for withdrawing cash from ATM's) far safer than carrying cash and gives much better value than paying cash. I give tips in euros.
Gav's suggestion is also better than exchanging cash.
You can always call in to see the beautiful San Stefanos Travel girls but you don't have to feel obliged to exchange your cash there.