BA2 is usually diverted to LGW as it's where the aircraft is maintained and the flight deck and cabin crew are also based. With the second flight now gone, there is no wiggle room. If the flight doesn't land at LCY from NY within a reasonable timeframe, then it's cancelled. The second aircraft - EUNB was on a one year lease (dunno if it's still there - I see it on flightradar24 occasionally) with Titan based out of Stansted. Looking at it's lack of use, I suspect it may not have been chartered as much as predicted. I also haven't seen BA use it at all, though if BA2 can't land - then neither can EUNB.

I've done BA 1 - both on Concorde and LCY - Shannon - NY. It is unique, and a one off experience - almost like being in a private jet with three gatwick crew who deliver a good product. Given it's departure time, the worst thing that will happen is you taxi to LHR and get a mid afternoon flight. Plus - pre clearance at Shannon is hassle free and a pleasant experience