A B767-300 on that route wouldn't allow you to carry much cargo with a full pax load. Johannesburg is not "twin" friendly at the best of times but even on the southbound service from Gatwick the aircraft will be approaching MTOW or Max Fuel due to the reasonably high-density configuration Nationwide offer (no F or C)
carrier isn't managing to fill their JNB
-Europe flights then they have a real problem! Demand is high when measured against the available capacity, which is controlled to protect SAA. Much UK-JNB
traffic transits over European hubs and the Middle-East, as much due to lack of availability as pricing - real "bargain" fares to South Africa are often difficult to obtain through normal distribution channels.