Harold Godwinson

The Thirteenth Doctor once met a King Harold, who she described as a "big fella" who was handy with a sword, and liked a good knock-knock joke.

The Doctor once told him a joke: "what do you call a knight who always cheats on a test? Sir Glancelot. (COMIC: Mistress of Chaos)

Behind the scenes
The Doctor may be referring to Anglo-Saxon king Harold Godwinson.