Hi,
KaptainKrunchie is correct. You could take the Turkish Skipper test or you should cancel the Turkish registration and apply online for a joint registration with the UK Small Ships Register. It's easy but You need a UK home and delivery address for the certificate. I assume both owners hold an ICC. If not, the ICC holder has to be on board as skipper. I suspect you will also need to obtain a 5 year Transit Log as a newly registered UK boat. That is another can of worms but the agents opposite Ege Marina can take care of the application. Finally, you will be able to fly the Ensign legally with the Turkish courtesy flag. Good luck!