I'm not quite sure that the whole picture is being presented here.
http://nationalprivatepilotslicence....20CPL%20TK.pdf
So they can teach for the NPPL including all its associated ratings and lets remember there are a significant number of microlight instructors who make a their living out of just that. A suitable qualified FI without CPL TK can do that.
Also upon completion of the FI course if the candidate is suitability qualified (300 hours TT) they can apply for a CRI rating. This allows them to teach for numerous ratings. Do the hour with an instructor flight and the numerous types of differences training that are available. Also there is nothing stopping them adding addition privileges such as aerobatics
I know 5 FIs without CPL TK. Two work full time and both used to work for the largest ATO in my part of the country. The other three work part time and seem to be fully booked.
Also there is nothing stopping any of these guys from doing the CPL TK at a later date and one of them did just that and he now works for a fair sized UK airline.