The OP is in the UK and so the relevant legislation is the ANO, Article 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.
(2) A person may act as a pilot of an EASA aircraft without holding an appropriate licence granted, converted or rendered valid under the EASA Aircrew Regulation when undergoing flying training, including solo flying training authorised and supervised by a flight instructor, in accordance with the EASA Aircrew Regulation as amended from time to time.
The question therefore turns on what constitutes 'acting as a pilot'. If the courts decide that, by manipulating the controls of an aircraft in flight, a person is 'acting as a pilot' then the situation as described by the OP is illegal.