Southend is totally inadequate for 20m passengers - Simple.
Runway, taxiway, apron, terminal, car parks, surface access you name it, physically not possible, even 6-8 million would be a huge challenge. Ok so you could exand, but how, and where more to the point!? The airport is boxed in on all sides by residential and industrial estates. To achieve this at Southend, you're just as well to lay another runway down 50 miles due west for the noise impact and number of residents you'd displace.
It's claims like this that makes me seriously question the crediability of Stobart to ensure SEN does have a future full of realistic prospects: an overspill to LGW and LHR, which to some extent is all STN and LTN are, except SEN were able to deliver the additional peak capacity which even STN and LTN couldn't. Plus how do they intend to embark on such expansion when their airlines are paying next to nothing?