This specifically targets the portion of SOP, which MPs are obligated to follow within rules - that forbids MP from wearing no headgear or other cosmetic headgear if they so desire. “MP’s must always use their department-specific headgear.” Why does this exist? It’s been a thing for years, and when…

As above, you do not have to wear headgear as an MP. But if you -do- wear headgear it has to be MP headgear