Certainly when I worked for MoD in flight test, test pilots assessing an aeroplane always logged P1, regardless of what seat they were in, and regardless of how many pilots were logging P1!
So, left and right hand seat, both TPs, both logging P1 - entirely in accordance with JSP318.
(For the avoidance of doubt, my military flying was either as a cadet pilot in a Bulldog, or a Flight Test Observer which is logged crew, but another column in the logbook altogether.)
G