That’s a tricky question, but probably Arthur I because Arthur is a legendary king, not a historical one.
I think he’s going with Charles III.
Maybe he should select a new one not one of his given names.
I vote for Athelstan the Bretwalda. (Bretwalda - Anglo-Saxon for Ruler of Britain.)
It would be a good choice particularly since Truss’s new cabinet seems to be devoid of Anglo-Saxons or even Celts!