This isn't specific to Rhodes as it will be true anywhere in the Eurozone, but as we've just returned from Rhodes this seemed as good a place as any to post.
In Lindos the exchange rate on offer was 1.21 euro to the £. I've just checked the statement for my Nationwide Flexaccount and I was getting close to 1.26. That's about 4% more for your money. If you don't already have a Flexaccount then think about it seriously before your next trip.