I checked something similar to this with the CAA a few years back, it was with reference to flying a small twin turboprop executive aircraft, I was operating as a co-pilot (pilot's assistant, call it what you want) and asked them the legality of logging those hours as P2 just for my own experience record and not for licence issue. They said that I could log it as I wished, and that they would even be prepared to allow either 50 or 100 hours to count towards a CPL issue if the Chief Pilot/Captain could confirm that I was contributing to the operation of the aircraft. Sometimes the CAA can actually be human and think beyond the rule books that people on here are so great at memorising and paraphrasing verbatim. your logbook is a record of your own flying experience. (edited to add-the original question was P2 on a light single, not much chance of getting anything for that)