Ian, I have posted this before, buy the Turkcel MiFi in Turkey, use it top up not a problem, then I can confirm that it does not need unlocking in the UK for 3Store network. I took it to a few shops looking to have it unlocked and they were not able to do it, but 3 store works not a problem. with 3 store you can register and pay online for Gb Data.
Buy one from the UK and take it to Turkey and it will eventually be blocked if not registered. If you want to go that route and buy in the UK will the MiFi work with a Turkish Data Sim?