That's an interesting take on the myth of King Arthur and I can see the connection to the King Arthur movie that is mentioned few times in other comments. I just somehow find it hard to connect the idea of King Arthur as the defender of a united Britain with the idea that he should be a commander from an invading empire.
I haven't looked into the Arthur legends in a long time but this post has inspired me to go back and open some books, if for no other reason then just to enjoy the good old stories that comes with the legend itself.
Look at from this angle the Brits maybe adapted the story from the Romans, he was stationed in Roman Britain, he was a high ranking military officer in command of an army and he defended Britain, the territory Roman citizens lived :)
It is a theory :)