To instruct for the PPL whilst holding a PPL on SEP Class aeroplanes, you will need:
1. To have passed the CPL exams.
2. To hold a FI Rating.
3. To work at an RF from a licensed or government aerodrome.
Some of this may change; in particular the requirements to have passed the CPL exams and to work from a licensed or government aerodrome. But not for some years, I would guess.
PPL, then experience, then FI Rating, then instructional expereince.....and then CPL and IR is the way ahead as Mike says. But tit can't be done with just an exemption, there would need to be an ANO change.