BA operate an A319 service from LCY to JFK via Shannon.
Actually, it's an A318, all business-class configuration, and it only has to make the fuel stop in Shannon on the westbound leg. They use Shannon because there's a US border pre-clearance facility there, so the pax can go through immigration while they gas up the plane.
The only reason the A318 can take off on the LCY runway is because they only fuel it up enough to make it to Ireland. There's no way it could make it at MTOW.
On the return trip, from JFK-LCY, they fly nonstop, as taking off at JFK is no problem at MTOW, and by the time they get to LCY they're light enough to land.