How can I change my Apple Account region back to UK?

It is not easy to make a mistake of that kind. You would have to have a residence in Turkey and maybe also a telephone number in Turkey and a bank account there. Are you sure you changed country? Maybe you simply changed region setting instead?

Here's how you can correct this:

Sign out of the iTunes Store [ Sign in to your Apple Account - Apple Support ] and sign in again to get the iTunes Store region to match the region to which your Apple Account country is set, as described in this post: apple id is only valid in us store - Apple Community

1. Go to: Settings > Account > Media & Purchases > View Account > Country/Region

2. Make sure it is set to your normal country.

3. In the "Media & Purchases" menu, sign out and then sign in again.

The above is a simplified version of that in the support document 'Change your Apple ID country or region' ( Change your Apple Account country or region - Apple Support ) except you shouldn't need to do the extra steps involved as when you have done a true country change.

Hello~ Click on below and review…

Change your Apple Account country or region - Apple Support

~Katana-San~

Dear Andrius, here’s how you change your region back to the United Kingdom on your iPhone or iPad:

First step, you open the settings app.

Second step, you go to General.

Third step, you go to Language & Region.

Fourth step, you scroll down the preferred languages section, and tap where it says “Region“

And fifth step, you select the region “united kingdom”