Having been out of the RAF since 1977 and having no access to JSPs I don't know what the rules are. How anybody could be "Required" to travel business class on a predominantly trooping flight isn't quite clear to me! Will the peasant class get upgrades if the posh seats are vacant?
I'm led to believe the larger legroom seats are partly there for minor leg injuries. Gives folk more room without using the aeromedical rig. They may also available be available to "sell" on to other govt departments, organisations and civilians. Thus defraying some of the cost of the airbridge.