The idea of a trial leson is to give somebody the oportunity to discover what flying is about, not for pleasure but with the aim set in learning, that's why the CAA allows this kind of flights to take place without asking for an AOC to the PPL training registered facility.
What is happening is that most of the trial flights are indeed pleasure flights, if there is somebody from the CAA checking one of them and he sees you are not brieffing the STUDENT, he'll for sure consider it is ilegal, on the other hand , brieffing the STUDENT does not mean it is legal in front of a court as you point out, but for sure you stand much better chances of not getting shafted in court if you have brieffed the STUDENT.
As is often said , better safe than sorry, cover your back, brieff them even if you positively know they wont ever ever come back for flight training
What do you think of the trial flights where the STUDENT comes along with his friends or relatives?, i mean when they get along in the aircraft for the flight, does anybody know what is the CAA view on this matter?