One significant fact is that all FIs are required to:
Meet the knowledge requirements for the grant of a JAR-FCL Commercial Pilot Licence (Aeroplanes) as per AMC FCL 1.470(b).
which means that you must pass the CPL theory exams.
Things may change in the future for instructing at PPL(A) and NPPL(SSEA) level, but don't hold your breath!
Regulations for NPPL Microlight and/or SLMG instruction are different and perhaps you might care to explore that avenue?