According to a previous thread, PFA coaches and assistant coaches can receive £25 per flight "expenses" without it being considered aerial work - hence a PPL holder can be a coach. This has (according to the PFA) been agreed to by the CAA.
With this precedent having been set, maybe part time flying instructors could also receive £25 per flight "expenses" without it being considered aerial work and hence not counting for FTL. This would have the added bonus that the "expenses" would not be remuneration and thus would not be taxable. It is of course a grey area and CAA clarification would be needed.
The whole thing is bureaucracy gone mad anyway. The purpose of an FTL scheme is to prevent undue fatigue. I would like somebody to explain why an hour of paid insructing on a day off is more fatiguing than an hour of unpaid instructing.