According to our company OM part D (JAA but non-UK) one is allowed to log PICUS only when flying with a training captain after specific experience requirements are met
That's the thing with the UK though, if you didn't get PICUS time on the right seat with fATPL, you wouldn't have the hours to unfreeze your licence if you started your career with the minimum required 150 hours PIC (or whatever it is for Integrated guys).
P