AIUI, for the UK PPL you do have to enroll in a flying school; there is no freelance instructing possible on the PPL.
The plane needs to be either wholly owned by the student or his spouse (there is a reference to this somewhere in the ANO) or, if group owned, needs to be maintained to Transport CofA requirements. These requirements come out of the fact that the instructor will be paid. If you somehow managed to wangle it so he does the flight training for free, then the maintenance requirements are just the Private CofA ones, even if group owned.
There is no legal requirement on the ground school - you can get anybody (an experienced PPL would be a good one) to teach that, and there is no scale of charges. Whether the school you enrolled in will tolerate ground school being done by outsiders is another matter - the standard "airfield politics" thingy
