"Trial Lessons" are just that, a lesson that is a trial, it could be anything that a FI is entitled to teach however; it is usually based on Ex 3 in the PPL syllabus, Air Experience.
The CAA legal department did try to define a "trial lesson" some years ago, but gave up as it was too difficult.
The real issue is what is the purpose of a trial flight, is it to introduce a potential new customer to flying lessons, or to offer a scenic tour culiminating in a lunch somewhere.
In the latter case, the customer will expect a higher standard of safety than can be guaranteed in a flying club hence the requirement for an AOC. Flying clubs are exempt from the requirement to hold an AOC, and potential students must recognise that there is a greater level of risk associated with learning to fly, than they would expect if embarking upon a public transport flight.