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. 150 hours of these flights are flown and recorded in a separate syllabus as part of commander course prior to commencing type rating for left seat. Decision making in operational matters is emphasized (tankage, ground handling, de-ice, etc).
All other flights are logged as "Co-pilot".