Askeladd is a half-British descendant of the legendary King Arthur and his birth name is revealed to be "Lucius Artorius Castus", the name of a real Roman soldier who some think was the "original" King Arthur because he once lived in Britain. Presumably "Lucius Artorius Castus" was in fact King Arthur's real name in-universe and Askeladd was directly named after him. Based on this theory, the King Arthur movie from 2004 (before the manga started) brought it to a wider audience. But the movie was far from historically accurate, despite selling itself as the true story behind the legend. And the theory itself is a very weak one, one point being the real Artorius lived hundreds of years before King Arthur supposedly did. The only possibly solid link between Artorius and King Arthur is the name.
Vinland Saga already stated that its Arthur was the leader of the British against the invading English, which follows the traditional legend. Instead of "Lucius Artorius Castus", Yukimura could have used the name "Ambrosius Aurelianus" after another guy who did just that, and is another candidate for the "original" Arthur. "Artorius Ambrosius Aurelianus" has a nice ring to it.