I've had a few editors complain that I use British slang or words when writing for a US market, and the reason for that is pretty simple. British authors form the backbone of my fantasy experience. There might be a fantasy author that hasn't read J.R.R. Tolkien, but I wouldn't trust them. C.S. Lewis is nearly as well known, but hasn't had the same level of pop culture success as the Lord of the Rings. I think that's kind of a shame.
Tolkien was a genius, but most of his characters were very black and white. The bad guys were bad, and the good guys were good. For all that Lewis is celebrated as a Christian author, his characters have more nuance, with many existing in the realm of moral relativism. Lewis introduced me to the idea that you can have protagonists the reader doesn't like and villains who genuinely think they're doing what's right (or at least what's right at the time).
Lewis also provided the lesson that the world is just as important as the characters. The children come and go from Narnia, but the world doesn't stop while they're gone. Narnia also continues even when the mundanity of life turns children into adults. New characters emerge to take up the wheel, allowing new stories to be told from different points of view. This particular lesson will come into play in Last Fool Standing. The world moves on even while Kovos seeks the city of death, and we'll peek in to see what's going on.