Just watched the mayor's video and he says "...you can also go into Dublin and fly transatlantic with United".
I checked on united.com, you can book the flight already but a £400 DUB-JFK flight goes up to £5,000+ when you start at Teesside. This is an interline on an LM number rather than a codeshare which may explain the pricing as the DUB-JFK legs went into the full fare Y class. If a codeshare appears then the price should drop to normal levels. Alternatively, this could just mean that the full pricing has not yet been loaded. Nothing via LHR with United yet.