R280:-
This is just a subsidiary rank. He is already a Colonel, and got married as such.
Well yes, but he is now a substantive Royal Air Force Squadron Leader (whether rightly or wrongly). That might count for rather more in his eyes than any honorary ranks (including Colonel of the Irish Guards) that he might be appointed to.
So why didn't he get married as an RAF Flight Lieutenant? I don't know, but the fashion and society magazine sales would not have been so great, I suspect, nor the TV ratings. Perhaps the bride had some input? It was after all her day...