Convert a few GBP into YTL here in the UK for any purchases when you land and for tips etc. Then convert the bulk of your GBP to YTL in resort, the exchange rates there are better than at home.
You can either take folding money if you prefer, or take your debit card, you will find plenty of ATM's. Just ensure that your card issuer knows that you are overseas. Also check their charges. The best is Nationwide.
Euros get mentioned a lot, simply because there are a wide variety of European visitors to the region, and it is easy for businesses to display comparative prices in Euros.
Hope that this helps.