Geoffrey of Monmouth knew a good story when he found one. He could have produced great movies.
Both King Arthur and the historical figure and genocidal a-hole Boudicca has, ironically, been co-opted into the supporting myths of the post-Conquest monarchy of Great Britain.
It's not unlike the mythical Scottish resistance to Roman rule (it never happened) and the ridiculous "battle that stopped Rome" cooked up about the miserable failure, flash-in-the-pan turncoat Arminius, adopted as if he was ancestral to modern Germany, which instead is descended from migrants who replaced those people from whom Arminius sprang.
But yeah, Mary Stewart, author of “The Crystal Cave” and its sequel, did a magnificent job telling an entirely inticing piece of fiction based on the Arthurian legends, and in her afterword, notes that Geoffrey’s name was mud with historians, but as a source of great stories, he was (if memory serves) “terrific”.