JFK needs a daily service with a good business offering.
LAS is a leisure route, and is operated from LGW by the same "beach fleet" of B747-400s that operate the current MAN-MCO and BGI. So MAN-LAS would be a good fit for the current model, not sure if the B744 is too big though. I don't see it as daily though. The LAS market is there and has no competition.
JFK is a different animal, up against CO and DL they'd have to run daily. The 744 is I think too big, as is the A346 which leave the A343.
Virgin actually said they'd fly GLA-JFK if BA ever left. ( Virgin say a lot ). Until the 787s arrive, they are focused on LHR. Hope that makes sense.