The £10 or 15 Euros issue has always been a bit contentious but you can't really expect the authorities to keep changing the price as the exchange rate fluctuates. And you also have to bear in mind that they don't accept coins so it has to be in multiples of £5 or 5 Euros.
If you do the Kas - Meis run remember, you have no choice, you have to pay in Euros.
Oh, and they don't accept £5 notes in the Duty Free shop at Meis, because the Greek bank on the island won't accept them.