FatFlyer,
Where does that quote come from? I've searched
CAP 393 for your quote, but can't find it. You've said that it is "Rule 28". Rule 28 relates to VFR flight outside controlled airspace, and is not relevant. Wondering if you perhaps meant Article 28, I checked that too, and found that Article 28 is the requirement for a Certicate of Release to Service for non-EASA aircraft, also not relevant.
The closest I can find to "relevant" legislation is actually Article 50:
50 (1) Subject to the exceptions set out in articles 51 to 60, a person must not act as a member of the flight crew of an aircraft registered in the United Kingdom without holding an appropriate licence granted or rendered valid under this Order
So you need a license to act as flight crew.... but a passenger would not be acting as flight crew, they would be acting as a passenger. There is nothing to prevent a person who is not a member of the flight crew from handling the controls so far as I know.
Since I can't find your references in UK legislation, I wonder if we are talking about laws in different countries? For the avoidance of doubt, everything I've said relates to UK law.
FFF
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