These things have been done to death here.
The reason nobody can settle it is because - apparently - the CAA has never prosecuted anybody for what one could describe as debatable cases of aerial work. They also refuse to be drawn on where they would draw the line.
They have prosecuted breaches of Article 115 (140 in 2005 ANO) in cases where there was obvious and easy evidence of aerial work, perhaps from disgruntled individuals. But that is foreign reg aircraft, not G-reg, so always provocative.
They have prosecuted blatent cases of aerial work in G-reg, but to get this you would have to ferry a load of fare paying passengers in full view of a severely p*ssed off AOC holder.
IMHO the person to ask is your insurance broker, or make sure you are a named pilot on the owner's policy.
If you are not a named pilot, then you are not insured yourself. The owner is, but you are not. Not many PPL students and self fly renters know this!
In most of these scenarios, insurance is what really matters, not the CAA.