Bluwise, you have one of the "bad" cards for spending abroad, but not one of the Worst Five. Everyone using cards has to pay VISA/Mastercard/American Express/Whatever service charge for using your card, but this is trivial. On top of this, if, say, you are buying jewelry for 365TL (= £100) then NatWest will hit you with an exchange rate load of 2.75% (= £2.75). The good news is that you won't be hit with a spending penalty (for goods) on top of that - some other banks charge you twice! So your £100 jewelry is going to cost you £102.75+VISA. You might not want to get too worried about this. If you were paying a hotel bill for 4 people at £1000 it would cost you £27.50 more and you might want to think about this. If you used your NatWest card to withdraw TL from ATM and paid cash for the jewelry it would be slightly cheaper £102.00+VISA (or £1020+VISA for the hotel bill). [Note for the very technically minded you could, by drawing out the full amount allowed by ATM/your Bank, you could shave a tiny amount off £1020+since there is a charge cap for Nat West].
I don't know what exchange rates Turkish banks charge because I have never used the "Settle up in Pounds" facility but my guess would be it would be around 3%. So you would be better off with NatWest. Under no circumstances would I accept the retailer making up the exchange rate and then telling me the bill would be "£107 but for you I offer £105, special value".
In short, if it was me, I wouldn't worry for sums at £100 level because it wouldn't be worth my time worrying about. Just pay in Lira on your debit card. If you're doing serious spending, or you enjoy getting the better of banks (that's my reason) then you might want to think of opening a "holiday account" with a different bank. You'll still have to pay a tiny VISA/Mastercard amount (well worth it for using their card service) just like you do now, but you won't pay any "extras". My card changes nothing to use ATM or buy goods and has an exchange loading of 0%.