The system used to be.....
Take your UK phone and passport to the Turkcell shop (on the Tasyaka bus route out of Fethiye where all the electrical shops are). They register it for you for free and within a few days it will work with a Turkish Sim card. This can be done for one phone per year (per uk passport) at no charge.
Don't know if anything has changed now (i did this 2 years ago)
