Not sure if all the info is there in your post, so.
You have an old Turkish phone, old Turkish Sim in the phone, just put credit on the phone and approx 1 hour later it is ready for use. Our friend came out last year after an 18 month lapse and was able to do this with his Avea sim in his very very old Turkish phone. no passport required.
You have a Turkish phone but no sim, you need your passport to purchase a sim card cost can be 30+lira but includes the credit/kontors
You have no Turkish phone, you buy a sim card (again cost of above) you need your passport, insert into you UK phone and it may stay open for 1 day or 2 weeks, you take that chance, you could be lucky. if it does last keep keep the sim, if you are intending to return later 6months, 1 year later why not go to any of the telephone shops and buy the cheapest 2nd hand phone they have, 40TL+ bog standard nothing special.
I registered online with Turkcell, I received the password, you have to be in Turkey to register, once done you can put the minimum of kontors on the phone when in the UK. make at least one call to any phone in the world during that time to keep alive.
I forgot my password once, so now I buy a 20TL kontor card (getting rare) leave the kontor card with a friend and when the time is right ask them to go online and add the kontor card scratch off number, a message will come through to your phone where ever in the world you are and it is all legal ready for the next time you step off the plane.