There is a clear difference between competence and legality.....
No doubt the airline co-pilot, ex-A2 who is now a display pilot is entirely competent to have taught military students aerobatics, for example.
But it isn't legal for him to teach civilian pilots, of unknown aptitude, to fly aerobatics without having first gained the relevant civil qualifications.
It was much easier for me years ago when I decided to become a civil FI. First I had to regain my PPL, apply for IMC and Night Ratings, then pass ATPL Air Law for my R/BCPL. Then fly with the CFS trapper who was visiting the other UAS at the time, to meet both 1179 and FI test requirements. Then send off for my FI Rating and R/BCPL at the same time....with a significant cheque after first arranging the civil medical. Later I added a BX examiner authorisation and eventually converted to ATPL (after flying 1 trip with a CAA IRE observing an MCT trip in the VC10), with SEP Class Rating, IMC Rating, Night Rating, FI Rating and FE authorisation.
The cost of keeping all this valid, coupled with a downturn in student demand and my availability, eventually forced me to pack it all in. JAR-FCL had forced me out of the market.
The instructing and examining I did in the PA28, plus the odd aeros session in other aircraft, was nothing like as demanding as UAS Bulldog instructing - and my A2 on the Bulldog was vastly harder than any civil FI test.
Regrettably, the civil world is paved with road humps, all of which cost a lot of money to cross. Does it make you a better pilot or instructor? Does it hell....
BillieBob, your JAR-FCL licence is now deemed to be a part-FCL licence and is no longer regulates under the ANO. See CAP 804. FCL900 is reasonably all embracing with regard to Instructor privileges.
Anway, why not get yourself properly qualified in accordance with part-FCL, rather than operating under your barrack room lawyer 'where does it say I can't' nonsense?