Jet 2 to JFK - unlikely
Your enthusiasm and speculation regarding Jet 2 and JFK is getting a little out of hand. Just because they are getting 757s does not mean they are going into long haul. Further speculation about them getting into bed with Jet Blue is also going into the realms of fantasy, for these reason alone
1 Loco long haul has been tried before and has not been exceptionally successful.
2. Loco TransAtlantic connecting with Loco US flights will dilute rvenue for Jet 2 across the Atlantic, making it less profitable, if they were to consider through fares. At the end of the day it would be 2 separate airlines concerned, Jet 2 and Jet Blue, not the same one offering point to point fares. But...
3. Locos do not guarantee making connections - flight to flight, if you miss your flight you lose your money. They would have to start offering compensation or reroutes, or hotel accommodation if the flight from Leeds was late for any reason. 80 or so missed connections would prove a little expensive, and that would eat into any profit margins big style, and Locos just don't do that.
So, sorry to put a dampener on things, and as much as I would like to see it happen, I think in reality it won't.
But I have been wrong before!