I do believe the Industry would benefit greatly from having experienced pilots (instructors) going back to ab-initio instructing. Consider writing to the CAA to obtain details of exactly what is required to renew your FI with SEP. I believe each person would be reviewed based on their experience, so training required could be a lot less than you might think!
An ex-colleague of mine did this about a year ago and was quoted 'training as required' to meet proficiency plus an AoC with an FIE, which resulted in less than 10 hours total training. The FIE, I believe, did the test for a minimal charge as a courtesy, and the school which was interested in his services paid for 50% of the costs for a commitment to work as an instructor for 12 months...sounds like a common sense approach to me!
As I have mentioned in a previous post, flight pay rates (and even basic salaries) have definitely improved in the last year, so I don't think it would take years to recoup the outlay. I do know a couple of schools that are eagerly looking for mature pilots to fill a current void if anyone would like to PM me for details (I think I am allowed to receive these now?).