You can top up from your Turkish bank account, if you have one.
We were last in Turkey October 2013 I used the phone last on 7th October, I knew I would have to top up by April 2014. So I used the bank from the UK online.
Unfortunately I forgot to put the phone on Roam before leaving Turkey in October, yet it appears that just by topping up has done the job. We stepped off the plane 3rd June and it has worked fine.
To get the Turkcell phone to work in UK and do top ups by kontor card I used to register online at the turkcell website, you only can do this when in Turkey, unless it has changed.
Prior to me topping up online through bank I used to give a friend 20tl and they had my telephone number and a day or so before the 6 month lapse I would ask them to go or when they were in any shop buying milk or bread to do the 20tl top up. This has kept my phone open for the last 6 years.
Hope this helps.