CAP 804 Section 1, Part E, p 15, 10.2 states, amongst other things, that a Co-pilot may not log as flight time any periods during which he does not occupy a pilot’s seat.
The reference is to the "designated" Co-Pilot.
A pilot supervising Co-pilot activities is a designated "supervisor" and not the designated "Co-pilot" and trherefore may in accordance with F(ii) log that time as 'P2'.