Suggest you read
http://www.moneysavingexpert.com/travel/cheap-travel-moneywhich will tell you everything you need to know and more. Currency conversion is a minefield but you should be able to do better than a high street bureau de change and certainly better than the sharks at UK airports.
Remember that on Symi there are limited opportunities to use credit cards in the way we're accustomed in the UK - most tavernas, tourist shops, taxi boats etc aren't geared up for plastic. So the info on how to get the most euros for your pounds, whether at home or abroad, is what you need to home in on.
Jan