Rivet - I've flown through STN approx 200 times. Yes it feels cheap, but a lot of cash has gone into it over the last few years to tart it up. Stansted's airport infrastructure is fundamentally quite good.
If BA can offer fast track security, there are plenty of people in STN's catchment area who work in London and earn well who would happily pay to avoid Ryanair and live too far from Heathrow or Gatwick to make BA normally viable.
I used to live in Swaythling, about a mile from SOU and want SOU to succeed, but still don't see a compelling reason as to why BA would do better financially from SOU compared to STN.