A "Trial Lesson" is exercise 3 of the PPL Sylabus (otherwise known as the Air Experience Flight).
If the student is not folowing a course of instruction then they are not a student at all.
If they are not a student then IMHO we depart down the murky avenue of Public Transport.
Perhaps I'll start up regular flights from Bristol to Birmingham and get the people who pay to say they were on a "trial lesson".....that'll remove the need for an expensive AOC. They can even steer the wheel while I eat my sambos since I won't need to spend any money on autopilot repairs
I am not aware of any situation where an unqualified person can receive instruction outside of the RTF/FTO/BMAA/BGA system.
RTF registration is free.
Regards,
DFC