Like Tiger_Moth can't believe this has gone on for 6 pages.
Simple enough question asked about pax handling controls, with PIC not instructor.
Yes go ahead, not a problem. As stated give a proper handover brief and do this at a safe altitude until your pax has gained some experience and confidence.
However, your passenger can't claim the hours towards a licence. Time could be noted as SNY or OBS (supernumery or observer) with the time going in "any other flying" if they wished to keep a record.
I am not aware of any written official law (ANO etc) to prevent this, writing to the CAA isn't going to help anyone.
Remember though you are the "Pilot in Command", if your passenger does cause a problem - it's your fault!