When I was a guliable cadet a number of years ago serving with my local air training corp we had an instructor exactly like this...
Told everyone in the cadets that he was ex-RAF regiment who had then joined the TA Paras, most of my officers were all non-military guys so didn't really know much.
My dad was on leave from the army and came to pick me up one night when he got talking to the guy about who he had served with. My dad, being a para at the time, asked him about the paras when he admitted he had actually served with either the rations or signals TA squadrons as well as the RAF Regiment.
Anyway... Me and my dad were down at a local TA Base and got talking to the RSM about general stuff when he asked about the instructor.
Turned out he had failed RAF selection joined the TA where he was later kicked out of for being a lazy b*****d