Also with regards to the suggestion that flights are cancelled just to save money if they are booked light- airlines have to be careful with their slots, especially at LHR and LGW- where slots are on a use it or lose it scenario. They have to fly the slot at least 80% of the time to keep hold of it for the future season, no excuses.
If they do cancellations for low bookings early in the season, then they leave themselves very open to problems later on if they have a spate of technical cancellations, weather cancellations, or even strike cancellations and cant fly the slot, and drop below the 80%, thats it, slot gone, and VERY difficult to get back!
So given the value of a slot, its a lot better to fly a plane with a light load, than lose the use of the slot!
BAATW