His promotion is to do with maintaining service parity with his KCVO, that's all.
I don't see why. CO HQ LONDIST - a Maj Gen is normally given a KCVO on retirement but is not promoted. Neither was Flag Officer Royal Yacht - a Rear Adm. So why Has he been promoted? Sounds like RAF aggrandisement to me, KCVO is a Royal award and nothing to do with the RAF prom system.
Does this make him the most senior Admin Branch officer ever?