LGW-JFK was a slot warmer for LCY-JFK.
Absolute rubbish. The decision to launch LGW-JFK was taken at a time when Silverjet and Eos were still going strong and were pulling some numbers of premium passengers from the Gatwick catchment area who, before their arrival, had used BA and other carriers' premium cabins ex LHR. If you lived in Brighton, Croydon, Tunbridge Wells etc and had to drive, you might as well to go LTN or STN which was easier and cheaper than LHR.
The logic was that a four-class LGW-JFK service would get the traffic back to BA by providing a service from people's doorsteps. Silverjet and Eos had both vanished by the time LGW-JFK started, but the BA service didn't outlast them by all that long!