Sam Sam,
Don't see how they can extend the runway, one end of it there is the M27, and the other end of it their is an active railway depot. No, no one is rated on the E195 ... well maybe some JetBlue guys because I think the 190/195 is a common type rating.
As for other low cost airlines ... I think Air Berlin bring in a 737 from time to time but thats about it and it must be performance limited due to the runway length ... think Flybe said that they could never operate the 737 from SOU. The other low cost airlines are at Bournemouth ... think that is served by Thomsonfly, Ryanair, Easyjet to Geneva ...