What I do for flights where I'm legally a passenger (like this one), is to fill in dates, airfields, name of PIC etc, but leave all the times columns and similar blank. I then make a note in the comments column that my co-pilot did the flying. I do this on long trips, eg to the continent, so that I've got a record of where I've been. I also do it if I get to try out an aircraft for a magazine article (I do this now and then), but the person I'm with isn't an instructor, so I can't legally log it.
The CAA have never complained as yet.