I think you'll be fine but to be on the safe side I would have a debit card with me and try to put a few pounds emergency money in your account just in case and as someone suggested it's not a good idea to carry loads of cash, there are ATM'a everywhere. You have 2 meals a day in your hotel so you can get bread, cheese and salad and fruit in a super market for a picnic lunch or venture into a back street locanta for a cheap lunch. These back street places are where the Turks eat and you can have pide, toasted sandwiches, donner kebab or soup at a low cost. As has been mentioned a couple of the boat trips would be a good idea -10 pounds and they give you food and drink. You are sensible not to want to go shopping, I don't think the bargains are there like they used to be and dragging myself round shops in the heat is not my idea of fun. Where your money will go is on drinks. Save a little by getting 5 litre bottles of water or larger from the super market and decant it in your room, you should be drinking a couple of litres each a day. A 1 litre bottle from your hotel bar could be as much as 5 litres in a supermarket. I always have a small plastic funnel for that. Have a couple of romantic evenings on the beach alone with a few cans from the supermarket or your bottle of duty free. It's actually a really lovely thing to do, if you are lucky you might get a full moon. I'm assuming you are fairly young, my kids and their friends who are early 20's always take a few drinks to the beach instead of going to bars and always meet groups of young Turkish people often with guitars. They are very friendly and keen to speak English. They reckon it's much more fun than sitting in a bar.
I usually manage on 20 pounds a day and that is self catering but I have been many times and know how to keep prices down there. I don't drink a lot of alcohol or eat huge meals. Any way have a great time.