Joy Rides and AOC's
This issue stops being theoretical when there is a crash and the ensuing stewards enquiry kicks off, ably assisted by insurance company lawyers looking for reasons not to pay up for the owner/operator/pilot/hull/pax laibility claims etc. I have seen that happen too many times in the past 20 years.
There are ways that pax can pay, but ensure the small print in your insurance policy will come to the rescue if you have a bad day - as that may well cause more of a problem than the CAA.
The CAA leaflet about Fare paying Pax is too generalist, and does not cover the ground in sufficient detail - probably for reasons that we all know and suspect to be true.