I think you will find that seats on the LCY-JFK flights are being held back for the mainstream market they are aimed at. These people, in high finance at both ends of the route, rarely decide what they are doing more than a few days ahead - they are the classic full fare market that keeps BA going. BA are well aware commercially of all this, and want to have the right people, long-term supporters, on the flights in the opening days and weeks, rather than one-off travellers just there for the novelty. They can be used to bring the numbers up to 100%. I believe the limited number of seats offered for sale in advance for the first weeks have indeed mostly gone.