Originally Posted by
seawolf
EWR is not the same as LGA/JFK. EWR fares are totally different than those to LGA/JFK.
Right. As you noted, in fare construction, LGA/JFK will show up as NYC. That isn't to say that all fares are valid for both airports.
If the OP's ticket is written "X/NYC", and the flights depart/arrive within 12 hours of each other, 1 SWU is my vote.
If the OP can't resolve this with reservations, it can always be taken care of at the airport (a risk, no doubt) but I recall reading that the LHR check-in staff is pretty good.
The EWR thread is totally unrelated to this question.