I was wondering.. Is Peter Dinklage (Tyrion Lannister) immitation of a British accent good enough to pass off as a brit? it doesn′t sound very natural to me

It's extremely good, but rather 'mannered' - that is, the style of someone consciously putting on an upper-class English accent for the theatre.

Because ts not a British accent. Westeros is not Great Britain.

Having seen Dick Van Dyke in Mary Poppins, I consider that you have absolutely no right to complain about this - hopeless parody though it may be!

English accent, not British.

I am not British. But I agree, his accent sounds kind of "forced"