George R.R. Martin has stated that he based his stories on the English Wars of the Roses.
The geography of Westeros, in A Game of Thrones, is recognizably England. At least, it's inspired by the country. There's even a wall, which is patently based on Hadrian's Wall, in the north.
Winterfell is Northumbria, with an option on Yorkshire too. Dorne is an amalgamation of Cornwall and Devon. Riverrun read like a description of Cheshire. The story doesn't take us to Casterley Rock, but I'm expecting the Marches. Its rulers certainly act like the Norman Marcher Kings.
Even the races living there can be ethnically allocated. The Andals are the English. The First Men are the Celts (with a bit of Viking thrown in for good measure). Yet the folklore and customs are used so interchangeably, that it's probably better not to try and superimpose reality over the top.
For all of that, this is not Britain, unless it's set in some parallel world. It's as close as Tolkien's Middle Earth is to the same; and owes more of its inspiration to Lord of the Rings, than it does to actual history and folklore.
There's a common recommendation that, if you like Tolkien's work, you will like George R.R. Martin's work. That's fair enough and I'll repeat it here.