This question is raised from time to time. That Arthur was not a king during all or most of the period of his twelve battles is confirmed by HB which gives him the title “dux bellorum” in the following passage:
“Then it was, that the magnanimous Arthur, with all the kings and military force of Britain, fought against the Saxons. And though there were many more noble than himself, yet he was twelve times chosen their commander, and was as often conqueror.”
This is not surprising since during these campaigns he may well have been in his early to mid-twenties. That he eventually became king of Dyfed is shown by his entry in HG 2.
The Annals of Ulster has the following statement under the year 467:
“Death of Uter Pendragon, king of England, to whom succeeded his son, King Arthur, who instituted the Round Table.”
Hennessy pointed out that this comes from only version B of the annals and was done in a later hand. This explains the anachronistic reference to an English king and a Round Table. The date for Arthur’s coronation is, clearly, incorrect and may have been the result of a copyist mistaking the date cccccxuii to be cccclxuii. This is close to that given by the AC for Badon as opposed to being much earlier providing evidence that Arthur attained kingship towards the end or soon after his twelve battles.
 Hennessy, W. M., 1887.