Delta T - Noggin will be far more knowledgeable about this than me, but a 'Trial Flight' must include a ground brief and some element of flight instruction - even if it's only to consist of Air Experience. Hence it must take place in an aeroplane with a Transport(Passenger) category CofA from a licensed or government aerodrome flown at least by a FI(R).
There should be no reason why a non-paying passenger may not come along on such a flight if instructor, student and passenger agree - but the passenger must not be charged anything for the flight either by the pilot or by the student - or by anyone else. The 'free flight with every £25 cup of coffee' scam has also been tried in the past - it is 100% illegal!!
A while ago the CAA were, quite rightly, red hot on stopping 'illegal Public Transport' being conducted under the guise of 'trial flying lessons' - some cowboys were stretching the definition to absurd limits. A 'joyride' (in the old sense) is emphatically not a 'trial flying lesson' - it is a commercial passenger flight for which the aeroplane operator, aeroplane, pilot and aerodrome must be suitably licensed and approved.