RAF Topgun, I can only comment on JEFTS, but in their case yes you need to be qualified as a civvy instructor, with a fair amount of experience and be able to instruct to a reasonable standard already. However there is a possibility that if you did your FI course at JEFTS (you would have to pay of course) and were very good they would offer to take you on. I personally would not do that as the possibility is slim and probably dictated more by external factors than by your own performance.