RW20
A bit of a puzzler, so many factors to consider. Any significant increase is going to be driven by demand, the size of each aircraft (runway limitations apply) and the runway utilisation. If first two are maximised runway utilisation will need to increased by TWY improvements (i.e. the northern TWY). The name of the game being the ability for NATS to reduce spacing between successive arrivals, and increase departure rates (i.e. not dictated by need to backtrack either for 20 departure or after 02 landing).
The other limitation is the airport operating hours that dear old BAA signed up to (i.e. the Section 106 Agreement) to smooth the passage of planning consent when the airport was 're-developed.
Going to be a while before we need the Terminal 2 shown in the Airport's retrospective Master Plan. Would guesstimate at the current Terminal and Aprons (with some mods to the latter) are good for 3 million+. Not going to get to this with JS41s and ATR 42s. See how many PAX BOH shift with a fraction of SOU's movements.