ERM
Although flight crew experience is not necessary, they will probably be looking for something aviation related.
e.g. some folks I have known, active as ground school instructors at varous points:
ex RAF Engineer.
Engineer with a lot of time at Airbus on systems design and testing.
21 year old Aero Eng graduate.
mid twenties Aero Eng with a few years at Westlands under his belt.
(we advertised in national papers for the last 2, IIRC in the Times employment section, rather than Flight, in despair at the quality and availability of the people answering our ads in flight.)
If you rummage around the various schols websites, some have profiles of their intructors. (I discovered this whilst trying to track down an old work colleague.)
What subjects did you teach? You might have a possible in there, as I know some schools also run Maths / Physics courses to sort out folks before they begin the course proper. Might give you an angle.
You need to be able to demonstrate clear well structured lessons, deep enough knowledge level to cope a little bit deeper than the sylabus. You will probably end up with a much more didactic teaching style than you would use in a school. you can (a) get away with it, because the students are very motivated and (b) it is necessary due to time pressure to shoehorn the material into the contact time.
Regarding the JAA qualification, I can't help you with that. Haven't been involved with GS for a while, when I was there was only an approval system for CAP 509 (full approved ab initio) courses which involved being observed by the CAA, but for exempt training (most students) there was no requirement for any qualification on the part of the instructor.
Best contact the CAA or one of your contacts and ask them.
PM coming your way as well.
pb