Given the deployed nature of their roles, I wonder why TSW, TCW, MCSU etc don't also have the rank
I think I answered that one on page 1, TSW etc don't operate in fire teams or sections within infantry platoons as the Regiment do. The Lance Corporal rank was brought in to allow what would have been a senior SAC to command a fire team of 4-6 SACs and for there to be no doubt (visually and otherwise) who was in charge of that team.
Being deployed isn't really relevant, for the rest of the RAF the existing rank structure works just fine, at home or abroad.