If you mean biggest by "most important" then Gatwick is by far biggest for EZY, followed by Luton but it's Bristol that is the biggest outside of London, followed by Belfast, Edinburgh and then Stansted I think.
Liverpool and Manchester are actually two of the smaller bases but still have a significant presence at those airports. Ryanair is now bigger at both Liverpool and Manchester than EasyJet as far as I know (certainly at Manchester, in terms of passenger numbers).
I would think the most likely outcomes would be FR operating MAN-SVQ or EZY serving BRS-SVQ but if similar routes haven't lasted in the past then maybe their won't be such routes. I guess it's a case of time will tell.