Its probably fair to say if you teach PPL you will never recover the cost of the qualification.
You do not need a fATPL to teach, but if you want to teach commercially then you would need to be able to teach for the IR if you want a reasonable wage. The cost of gaining this qualification has recently increased by about £30K as you now need 200 hours under IFR to even begin. This may have a knock on effect with the larger schools where there will undoubtedly be a shortage of such instructors. As an alternative, this may prompt a move towards the MPL where they can use ex airline pilots and the straight through FI will not get a look in because they won't have the required 1500 hours experience on multi-pilot aeroplanes if they have not worked for an airline!
The whole thing is an ill-conceived mess. The future for the professional FI does not look cost effective for those just starting out.