I am not convinced either way about this. Not enough info.
I would agree that if the whole lot is one flight without a landing, then the
whole flight is subject to the AOC and to whatever rules apply under the AOC.
This may or may not mean anything for the ability of a PPL in the RHS to log the time.
If the plane is multi crew, that will narrow the logging possibilities fairly drastically - the RHS possibly can do it on a PPL but will need the Type Rating (if any) for the type, and the scenery here depends on whether G-reg or N-reg.
If the plane is single crew then obviously anybody (your granny etc) can sit in the RHS but her ability to log the time as PIC is a separate matter and again depends on whether G-reg or N-reg.
The rules on logging have changed, IIRC (this is not my area of interest) in recent years; before that a plain PPL, in a single crew plane, could log FAA (N-reg) PIC time in the RHS without having the TR.
To probe this area further, especially for multi crew aircraft, one could post in the Bizjet forum here, for this is a subject apparently close to the hearts of many, including some in the CAA
I don't have answers, but we have so little information.
Could this be taken as an "employee benefit" by HMRC and taxed in some way?
I doubt it.
Firstly they would have to find out about it (and the only record anywhere will be the RHS's logbook entry, perhaps countersigned by the LHS if the LHS is an instructor).
Secondly, what is the actual benefit? It isn't a free ticket to Jamaica. The value of the logbook entry is nil.
I know for a fact that HMRC does not care about benefit in kind when a PPL flies on his employer's business, even though the chap logs the time and the time could count towards an ATPL (a benefit) or could reduce his own insurance premiums (a benefit) - provided that the flight is wholly and exclusively for the business. Perhaps an accountant practicing in this area can input here?
Regarding insurance, well obviously the logbook entry will be written up only after a successful landing!! Once, years ago, I had a taxiing prang with an instructor in the RHS and he never wrote up anything, anywhere. As far as he was concerned the "flight" never happened with him in the plane.