In regards to how the logistics of it will work, nothing is decided yet, as talks/negotiations have just started. If we were to operate as our MOA currently states, we could do the lgw-lcy-snn on day 1, nightstop snn and head to nyc on day 2. Currently, we can't operate long and short haul on the same day, so the MOA will have to be re-written if the decision to operate lcy-snn-nyc on the same day is made.
To class the whole lot as a long haul flight would be a way round that, some short haul sectors out of LHR are very short, for instance Bahrain - Doha. So I imagine there is nothing to stop them getting crew to operate LGW-SNN-NYC and classing it as a long haul, if they operated it as a W pattern.