Dear Visitor,
First, relax that your will will be valid and honored in Turkey, even if you write it in England, due to some international conventions/treaties.
I'll shortly write what you have to do to leave a valid will in Turkey now considering different posibilities..
-You have to follow the English law orders to write a will if you will write that will in England(this is a validity condition in Turkish law), then your will will be carried out in Turkey.
-If the assets that you want to leave are real(unmovable) properties in Turkey, you can also fulfill the requirements of Turkish inheritence law to write a will. This will be valid as well.
-If you will write this will in Turkey, it won't matter whether you will follow Turkish or English law systems. It will be valid in Turkey in both preferences.
I naturally don't know the requirements of English law system to write a valid will. But in Turkish law, you can easily write a formal will by going to a notary together with two witnesses. Meanwhile, don't forget to take a sworn interpreter
Best wishes