For several years I have bought € at the Post Office as they didn't charge commission and the rate was good. They are still commission free but the rate is now very poor. My bank (Barclays) is now commission free and offers a much better rate. On the day I bought € it was €1.1994 against €1.1741. With the recent decline in the value of the £ we need all the help we can get!
For those with a Nationwide credit card (which also charges no commission for purchases abroad) I found their rate (which is the commercial rate rather than the tourist rate) was about €0.07 better than buying € in the UK. If you do use a CC abroad make sure they charge you in €, because if they convert to £ the rate they use is likely to be poor.
This thread could also be used to share experiences of the ATMs in Plakias, should anyone wish to do so.
Mike
We went to Paros in June and I took €100 with me which I got from the Post Office at the pathetic rate of 119 to £. I then used my N/wide
debit card to withdraw cash for the whole holiday and got between 127/129 to £ in Greece. Now, thanks to those who replied to my earlier query about ATMs in Plakias I'll take my leftover €s from June for the 1st day or so and draw all my cash there. If you use a N/wide
credit card in an ATM you will get the same rate of exchange without any transaction charge but you will pay interest from the date you draw it out until you pay it off.