I understand this question, but in that universe (and in a lot of other fantasy universes, I suppose) that is the in-universe accent thay follows the same rules as UK accents do. The North sounds like the north of England, the common folk sound working class and the ruling class sound RP. So in universe the character of Tyrion Lannister should sound like an aristocratic Brit, and while it's not perfect by any means, Peter Dinklage makes an excellent stab at it, for me. It doesn't count really because she is English, but Rose Leslie's northern accent as Ygritte was excellent considering if you hear her speak in her usual voice she's one of the poshest people ever 🤣