Raol Dahl was Norwegian. He had British Citizenship as he was born in Britain but he was Norwegian. His inspiration, his imagination came from his upbringing as a Norwegian.
Analogy, if a dog is born in a a pig stye, it doesn't become a pig.
His books are great. The horror and twisted stories are a reflection of the ugliness and horror he witnessed and experienced in the British boarding school system. Dahl reveals the sadism of one of the headmasters who later was to become the Archbishop of Canterbury* in his book "Boy."
Geoffry Fischer, Archbishop of Canterbury.