As Wiggy states, there are a number of factors that are taken into account when determining the crew numbers. Not all long-haul flights out of London's airports require in-flight relief crew (JFK, ORD, DXB,LOS,NBO etc..). Some Authorities and airlines do not allow co-pilots to sit in the LHS and therefore the Captain has to occupy the seat for the whole flight, even if 2 co-pilots are carried.