TOO, flight instruction is considered commercial flying: The old category “Arial work” went years ago.
From the CAA website;
Use of National Permit to Fly for flight instruction and self-fly hire
Guidance on using National Permit to Fly for flight instruction and self-fly hire
Flying instruction and hire of aircraft, is considered to be a commercial operation. Article 42 of the Air Navigation order (ANO) 2016
So, changing to a ‘Permit to Fly’ could have advantages. See the following documents:
ORSA No. 1393
Light Aircraft Association (LAA) Technical Leaflet TL 2.09 'Pilot Training, Introductory Flights and Self Fly Hire.'
A chat with the LAA could be worthwhile but again a licenced engineer may well be up to speed on this also..