I talked to someone who works for BA and they said the most difficult market to predict is the JFK==>LHR run. BA has a flight leaving on the hour, every hour between 5pm until 10:00pm. Most people flying business/first double/tripple book becuase they are unsure what time they will make it to the airport. The end result is some flights leave very empty and others way over sold. It is a very difficult market for them becuase of all of the duplicate bookings.