King Arthur is an English hero. Why is it a joke?

King Arthur is a hero of the British, but the reason for the joke is that King Arthur is an English hero in British legend and a national hero in the hearts of the British so far. Ironically, however, Arthur rejected the English ancestors who now offer shrines to him. King Arthur is the legendary king of England, the leader of the knights of the Round Table, and also a legend close to myth. It is English.