Article 50 of the ANO provides:
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.
ICAO Annex 1 1.1 ICAO Definitions:
Pilot (to) - To manipulate the flight controls of an aircraft during flight time.
You will note that article 50 refers to a person acting as a pilot, not pilot in command nor ‘the pilot’. You will also note that article 50 (2) makes reference to a person acting as a pilot when undergoing training, including solo flying not exclusively when flying solo.
Pulling ICAO, EASA and the ANO together, the strict interpretation is that a pilot is a person manipulating the controls of an aircraft. Such a person requires an appropriate licence unless undergoing flight training in accordance with EASA Aircrew regulation as amended from time to time. It follows that a passenger who manipulates the controls of an aircraft is acting at that time as pilot and needs to hold the appropriate valid licence unless undergoing flight training.
Whilst we know that in the real world this goes on all of the time and we have all allowed members of the family, pax etc to ‘have a go' and will doubtless continue to do so, strictly speaking it is illegal.
Whopity
the UK ANO only defines "Pilot in Command"
Yes – but Article 50 makes no reference to Pilot in Command, merely ‘a pilot’