Hi Sylvie,
Apologies for the delayed reply, I had to email our representatives out there, please see response below,
There is no difference between Turkish Lira or Euros, both are well accepted in Turkey. Sometimes and some shops do prefer Euro, but as I said there is no problem, if they pay in Lira.
Only regarding UK £, I would like to mention, that the clients should not change UK Sterling into Turkish Lira in England. They definitely lose Money. If they change currency in Turkey they will get a better exchange rate. Some of the tour managers mentioned this to me.
I hope this helps,
Kindest,
Ben