What I was suggesting is a base at Stansted alongside Gatwick, this would gain Norwegian extra passengers and possibly higher yealds
Wrong on the fundamentals here. All you'd do is dilute and cannibalsie your existing LGW operations. DI aren't going to serve both SFO and OAK from London for example, they're moving ops out of OAK to SFO, in much the same way they did from STN to LGW, as their analysis shows the latter airport is the more attractive option in market.
I think we're conflating short haul thinking with long haul.
EZY can serve AMS from LTN/STN/SEN/LGW easily, as that's something short haul point to point can support, four bases in London serving a mass market popular city destination. Also Jet2 coming into London short haul, capturing some ex MON and building out their own space. No one does this in long haul except the ever exceptional Emirates. Norwegian's issue of too many seats in the London market being sold below cost is not helped by adding more seats into that same London market, at another base, at *lower* costs.
You're attaching the short haul business model and assuming it'll work in that transatlantic and long haul market IMHO.