I shall take great joy in informing my instructor then who works for the CAA...
As has been demonstrated repeatedly, working for the CAA is no guarantee of an understanding of the rules.
You might like to refer your instructor to the CAA's own guidance on the subject. According to the 'Guide to Logbook Annotations', in LASORS Part A, Appendix B, the only times that PICUS (a.k.a. P1/s) may be claimed are when "Co-pilot performing the duties of PIC under supervision of pilot-in-command" (Case B) and "Pilot undergoing any form of flight test with a JAA or CAA Authorised Examiner" (Case J). Case B is limited to occasions when "
the flight was conducted in an aircraft having a Certificate of Airworthiness that requires its flight crews to include not less than two pilots" and so is not relevant.