Apart fromt the usual stuff, like the dog has got to be fully vaccinated, including anti-rabies and chipped. I'm fairly certain that one of the regulations requires that the dog is examined by a vet no more than 96 hours prior to export and that a certificate must be issued by the vet. That certificate must be legalised at the Foreign & Commonwealth Office AND the Turkish Consulate both of which are in London.
If taking the dog overland they must ensure that the correct documents are valid for all transit countries.
There is also the stress factor to consider. Not just for the owners but the dog as well. We were originally going overland with the dogs because we were worried about the stress that they would suffer whilst flying until a vet pointed out a few facts.
It could take 4 / 5 days to drive over, having to stop every couple of hours for exercising and watering the dog. Where would the dog sleep? The cost of certification ( £20+ extra per country ). Some countries require an import licence prior to travel. There are many factors to consider.
Apparently there has been quite a bit of research by RCVS that suggests that flying is far less stressful. Most dogs, once loaded onto the planes tend to relax and sleep throughout the flight. Granted, there's some stress involved at loading and unloading but they recommend it above driving overland.