I was in France recently and I must say, TMO were very responsible.
When I arrived in France, they sent me a text advising how much it would cost to make and receive calls and texts and how much per MB data (this was a little confusing, because I couldn't use data anyway unless I bought a booster, see below).
That said, I opted for a weeks data for £5 but it wasn't quite enough and I topped up twice for £1. Should have bought £10 in the first place.
Basically, you can't use data until you pay for it (get redirected to a payment portal if you try and surf for example) and when you use up that booster, you can't use any more data until you get another booster. Pretty hard to get caught out.
All in all, with the boosters and a couple of calls/texts here and there, my bill was over by about £15.
Enjoy the weather in Turkey
BTW, calls in France were £0.35 to make and £0.15 to receive if I remember correctly.