And to answer your question:
Yes, the accents you hear are, on the whole, authentic accents from different parts of Britain. There is no such thing as 'the pure British accent', however. There are hundreds of different accents within the UK. As is the case in Italy, a person's accent will depend on the region they come from. Accents vary enormously from one part of the UK to another, and may also vary depending on the speaker's age, class and education.
I recommend you follow the links to the youtube videos in the first post on the earlier italki discussion on this topic. They're quite amusing, and they explain about the different English accents in Game of Thrones. High-status families such as the Lannisters speak in very clear RP (the southern, educated English accent associated with upper classes) while the lower-status families use regional working-class accents from the north of England.
Regardless of their own origins, all the actors use the English accents associated with the status of their particular character. For example, Kit Harington naturally has an upper-class southern English accent, but when he is in character he uses a gruff northern accent; Peter Dinklage normally has an American accent, but he uses an British RP accent when he is acting in character.
Hello, Hanji. It's so nice to talk to you here again. ;)
I'm glad to have solved that mystery for you.
To see the contrast, take a look at this interview where Kit Harington is using his own accent, which is very clear:
<< For example, Kit Harington naturally has an upper-class southern English accent, but when he is in character he uses a gruff northern accent; Peter Dinklage normally has an American accent, but he uses an British RP accent when he is acting in character.>>
That's why! John Snow(Kit), his English is very hard to catch. Thank you for the information.
I like Sansa's accent.
There's a whole thread on this: