Aly
It only works really well to your advantage if you have an English Bank account that offers commission free T/C's. That way you only pay the commission when you change money in Corfu.
Exchange in England, at the mo, is e1.40 & in Corfu is e1.47. So, if I calculate correctly, £500 of com.free T/C's changed in Corfu brings you about e723 after their commission. This is against e702 if you can get commission free Euros in England. 21 euros in your favour.
About £14. - For a lot of people maybe not worth the hassle but for us when we rent privately, hire a car for 14 days, pay for meals, both buying cigs and alcohol to bring home we save about £20 or the cost of a bottle of Drambuie and a couple of retsinas at the Airport)
It's what you feel the most comfortable with really.
Cheers
Negg