I found the rabbit hole (but for nothing).

I'm using iOS in Russian, therefore my AppStore is Russian. I use "ограничения" - limitations(?), no one can change my profile or ID settings.

I wasn't able to download a book - Game of Thrones in iBooks. But then I download an app: "A Game of Thrones: The official community" HarperCollins Publisher LTD". It was in Russian AppStore. It was free with built purchases. It offered me to download the book, not for free, of course =) for 3pounds. OK, I agreed, and my Store has changed for UK, I think. Then my attempts to change the country back were unsuccessfully.

I was forced to unblock the limitations, and only then I could change the country back.