Try explaining it when it all goes wrong.
The whole issue of trial or introductory flights is fraught with unnecessary complication. The CAA legal department issued a guide as what may or may not constitute a trial flying lesson, and whether it was (then) deemed to be public transport or something else. The interpretation and definitions in the ANO verge on being ridiculous, and are unhelpful. To cut to the chase however, the holder of a professional pilots licence without a current FI Rating cannot provide a trial or introductory flying lesson. Equally, a pilot holding such licence and rating cannot provide a 'pleasure' or 'sightseeing' flight for reward/remuneration unless conducted in accordance with a valid AOC. The advice for Pilot Instructors is be very careful about the exact nature of the flight you are undertaking. When it all goes okay, you wont hear anymore, but the day when an incident occurs you will soon find out whether people were on board for a trial lesson or something different. You will be doing the explaining. The devil is in the detail.