candoo - just for your info an airline doesn't base its aircraft types solely on load factors! LHR is particularly complex and can sometimes simply be based on using whichever aircraft is available for the slot at that time. Eg a 767 arriving from LIS may be the next a/c immediately available in the right place for a MAD with only 55 passengers booked on it.
Obviously airlines try to operate types suitable for expected loads, but that is not feasible 100% of the time. Another factor is cargo load. BA's Tokyo services are hardly ever 100% full to warrant 2 daily 747-400s on the route, BUT the cargo revenue far outweighs an aircraft full of economy passengers! It sometimes means there is a heavy weight restriction on the route so to the passengers it seems as if the flight is empty, but little do they know the hold is full of valuable, high paying cargo! Food for thought.