I was told that due to the weather forecast that the spacing between aircraft landing was increased meaning they could not accommodate all flights. The short haul flights are the first to get cancelled as it is easier to relocate the aircraft to get back to the normal timetables.
Having said...
Had same problem with flight cancellation. Managed to keep hotel booking and return flight and cancel pod parking. Got good deal on train and with refund still in pocket.
BA customer care man told me he had several Brighton fans calling about cancellations so there must have quite a few of us...