In terms of proximity to London, SOU is not exploiting its existing infrastructure, being only 1 hour from Waterloo. National Rail Enquries tells me that LGW is 44mins from Waterloo. I would favour the one hour journey down to SOU and a quick walk into the terminal over the stress of LGW (and no doubt the North Terminal transit). If [FlyBE] were to sell rail+air tickets and heavily advertise in the Waterloo region (along with some decent post-work scheduling and routes) then I think SOU could pick up numbers. The reality is that not many people know of this connection (quicker than LHR, LTN and STN) in the Waterloo and Clapham area. It just requires marketing, no capex!
On that note - it is quicker to get from Euston to BHX than to any of the London airports...but that’s not for here!