Norwegian seems to be alone on all three routes
They're not alone on London-New York or London - Los Angeles.
Mind you, no one has tried this sort of thing since BCAL were flying B707s on ABC Charters. If their cost base is crushingly low it might work, they're only chasing tourists with this schedule, though with BA flying the A380 on LHR-LAX, I am not sure a LGW service brings much else to the table, although the route did well for Laker on a similar schedule, but with a much less competitive environment.
BA and VS both offer a good service from LHR, and I don't see this stealing too many of their premium passengers, so I don't expect much of a response. BA might review a LGW-FLL option but beyond that LGW-JFK/LAX don't work as well as LHR so why bother when chasing market share down the back.
It's something new that's for sure....