I've had a quick look at
LASORS at the
Guide to logbook annotation (Section A, Appendix B or page 56 of 610). In case B, Co-pilot performing the duties of PIC under supervision of pilot-in-command, the designation in the logbook is to be PIC U/S or P1 U/S and the time is entered in the P1 column. It then states that :-
Counted in full toward licence experience requirements subject to certification by the pilot-in-command.
Exactly what is meant by "subject to certification" I haven't got to yet, but to me it implies it would be a good idea to have the PIC sign your logbook just in case the PIC is unable to certify the time at a later date.
Regards
Stoney X