The CAA specify that a "Trial Lesson" should include some form of basic instruction in flying, and may not be just a jolly. Obviously open to interpretation. However I always treat it as an air experience flight (AirEx 3), brief accordingly and allow the student to fly the aircraft as much as they want.
Flying instruction is aerial work, and provided all the obvious legal caveats are covered, it is perfectly legal for persons to be carried in the back of a 4-seater during an instructional flight. I personally only do this on a trial flight, as the person in the back most likely has a financial interest if the student decides to complete a PPL.
XO