Have a look here.
http://www.caa.co.uk/docs/175/Lasors_Section_H.pdf
First you need to do at least the CPL exams (most people do the full ATPL exams instead as it isn't much more work).
You need to do an instructor course/test before you can instruct for the PPL.
You need to upgrade your PPL license to a CPL before you can get paid.
Is this why some 'sightseeing' flights appear to be publisised as 'trial lessons'?
Finally, I understand training flights must fly out of licensed airfields but does this also apply to 'air experience' flights for fare paying public?
The last 2 points are correct. Sightseeing on it's own would be classed as public transport.
FIS.