I had also though that the LCY->JFK flight made the technical/preclearance stop at SNN for fuel loading limits, but I also remember reading somewhere that there was another issue. LCY isn't a base for BA mainline, so the pilots actually report to work at LHR or LGW due to union rules and are shuttled to LCY, so they being their duty day too early to fly all the way to JFK.

Can anyone who's been on this flight confirm that there is a crew cockpit crew change at SNN?