Interesting thread, I recently approached AOPA about renewing my FI certificate and they were very helpful but frankly after looking at the requirements I was discouraged. Aged 62 I was originally an RAF Instructor on the Hawk with a ppl from a Flying Scholarship so did my first civil instructor training at Coventry with Piers and was tested by Hector Taylor at Nottingham in 1987. Piers was a first class trainer, Hector a very sharp old man who missed nothing and the CAA a sensibly run regulator.
I trained pilots at a variety of flying clubs while in the RAF, last renewed my FI with Ted Girdler down at Manston in 1995 and then dropped it as I progressed into civil aviation. I did my first Airbus Tour at Excalibur later in 1995 and flew with Hector's son. In 2004 I briefly renewed my rating at Humberside but did not instruct as I was busy and did not want to steal time off new pilots running through the self improver/modular route.
I have been a QSP on the Tutor since 2011 and for a number of years owned and flew my own tail-wheel aircraft. Today, on a 50% airline contract I was tempted to re-qualify but the process is a pain, the costs are irritating and the flying schools often badly run and exploitative. I love flying but the industry is ugly and the over regulation of everything associated with GA extremely tiresome.....just not worth it.