Those you refer to in DAP are serving Military wearing suits as they are seconded to the CAA.
The MET branch in the RN are serving Military personnel, the RAF are civilians (the met staff that is
).
I seem to remember that to wear a military uniform one needs to have done Military induction. I think it is a reference to the Geneva convention or something. This is where the RAF came unstuck with the putting civilians in uniform within simulators without them ever having gone through Cranwell. Cannot remember what they were called now. not Retired Officers, not sponsored reserves some other type of service.