On a big operation like this, there may also be a planned JFK standby requirement for down route sickness or other disruption.
FWIW there's no such requirement for this particular operation...BA Flight crew do not do standby downroute. In the case of JFK there are so many Flight crew slipping there that sickness/disruption would is handled by "je-jigging" in-situ crews' rosters, and/or positioning as a last resort.