You will make big loss by Exchanging your Currency in UK and Turkish Airports especially if you decide to change few hundreds sterling or euros.
Most travel agents and the post office in the UK / EU will tell you they have great exchange rates,that is not true.
You can shop with your own currency and if you in need of exchanging you will get better rate in Sarigerme.
There are no Currency Exchange Office in Sarigerme, however most shops will be more then happy to exchange your currency in to Turkish Lira, especially Jewellery shops and Travel agencies in Sarigerme will give you a good exchange rate without charging you commission.
Try to not bring fifty sterling banknotes with you as you may find it difficult to exchange as shops look at these banknotes suspicious.