The difference between a supersonic jet and and a subsonic jet is about 2x. Between NY and London that's a 3.5 hour flight vs a 7 hour flight (plus the overhead of security, traveling to and from the airport, boarding, taxiing, etc. which brings down the proportional cost).

By comparison, a boat would take 7-8 days. The disparity in time saved between supersonic and subsonic flight is pretty trivial in comparison to the time saved between a boat and a subsonic plane.