There is nothing in the law that says it is illegal to let a passenger manipulate the controls, you as
pilot in command are still the one responsible for the safe operation of the flight.
Actually there is:
UK ANO August 2012 Part 6 Article 50
50 (1) Subject to paragraph (2), a person must not act as a pilot of an EASA aircraft that is registered in the United Kingdom without holding an appropriate licence granted, converted or rendered valid under the EASA Aircrew Regulation.
Note it doesn't say "act as the Pilot in Command" it says "act as a Pilot"
I personally don't see anything wrong with a pilot, who is comfortable so doing, allowing a passenger
to try out the controls - but one wonders what the intention of the above wording was.