I can see nothing to indicate that you can instruct for a PPL on the basis of a Medical Declaration See the
CAAs web page. If you are instructing for a NPPL on a Microlight or SLMG then it appears you can.
A medical declaration (from 25th August 2016) is an affirmation of your medical 'fitness to fly' and may be used to exercise the privileges of:
A National Private Pilot Licence (NPPL)
A UK Private Pilot Licence (PPL)
A UK Commercial Pilot Licence (Balloons), non-public transport operations only
It is valid for flying with up to three passengers on board and in aircraft up to 5700kgs MTOW. It is only valid for non-EASA aircraft