In the oldest Welsh legends that mention Arthur his title is Ymerawdwr (Emperor). It certainly would be here as well because of the Roman connotations of the title.
Well, he's more than qualified for the title of "Imperator Britannorum" in my story, as in 502 He formally became the king of the Brythons, in addition to being Overlord of the English (as a result of mt.badon), and has considerable influence and respect among the coeling kings in the north.
Whether the Pope Symmachus, The Antipope Laurentius or the Eastern Emperor Anastasius would approve/recognize him is another thing