King of England
The inscription read, "Whoso pulleth out this sword of this stone and anvil is rightwise King born of England."
If Arthur existed, he wouldn't have been king of England, the land of the Angles and Saxons. Of course, even Mallory's work refers to "England" several times, as the name was current in his day. However, Arthur was more of a Brythonic/Celtic ruler in opposition to the Saxons, who were making inroads onto the island of Britain. It would have made more sense to use the name "Britain" throughout this movie instead of "England."