2 tours by anyone in Afghanistan after all is experience that shouldn't be wasted by organising ceremonial duties - there must be a herd of (useless) SO's who would love to do that
Whilst not equating operational service with ceremonial duties, its a big mistake to regard this job as a jolly which could be done by any "useless" SO.
Public duties in London include large ceremonial parades which are in effect state occasions, and present the public face of the army to thousands of people. The value to the country in terms of overseas tourist revenue must be very great, and can be compared in
PR terms to the Red Arrows, etc. Not a job where you want cock-ups.
I am reminded of an old Irish Guards officer chum of mine (now sadly deceased), who told me he was in charge of a defensive position somewhere up country in the Radfan in the early 60's. As they were keeping their heads down to dodge accurate incoming rifle fire from the dissidents, his sergeant, who was ducking down beside him muttered - "It'll be good to get back to London to do some real soldiering again, sir!"