Not wishing to be pedantic but why isn't this UK law considering this extract from JAR-FCL is quoted in Lassors?
Because LASORS has even less legal standing than JAR-FCL. UK aviation law is laid down in the Air Navigation Order, which includes no prohibition on a passenger handling the controls on a private flight.
by operating the controls one is acting as a flight crew member
Don't be silly! In a single pilot aeroplane on a private flight there can be only one crew member - the PIC. You may 'act' the fool but it doesn't necessarily make you one.