Lets look at what the Law says:
79.—(1) Every member of the flight crew of an aircraft registered in the United Kingdom and every person who engages in flying for the purpose of qualifying for the grant or renewal of a
licence under this Order must keep a personal flying log book in which the following information must be recorded—
(c) the capacity in which the holder acted in flight;
Thus if you have any operating capacity, it must be logged.
You can have two pilots in an aircraft, both rated to fly it, neither needs to be an instructor; if both have an operating capacity they must both log it! This does not fit any of the so called references.
The law does not define the maximum number of crew members, only the minimum number; it is the operator who deceides how many pilots are on board. Bear in mind most of the so called guidance is solely to allow people revalidating licences to easily determine if enough experience has been gained. At the end of the day its your personal log book; so long as you comply with Article 79 you can record it however you like. Of course if you are out of 90 currency you can only be Dual with an Instructor or Solo. P2 as already stated means nothing.