I'd probably think the majority go via LHR with BA/AA, in particular the 5/6/7 night holiday market where BA offer some very attractively priced packages.
If UA price it right then they could be onto a huge winner with the leisure market however of course they still need to make a profit and fill the premium cabins, something which BA doesn't really have a problem with filling meaning they can offer super cheap Y fares/packages to NY.
Time will tell, I guess we won't know how successful the route is until it is found out if it will be renewed/expanded for 2016 as even if flights go out full, they may not necessarily be high yield fares.
I do hope it is a success and it has been a long time coming - never saw it happening myself but good on NCL and UA for making it work and long may it last.
On another note regarding freight, will the 757 be able to lift much out of NCL and how big is the cargo space? I can't imagine it being enormous? Just curious