It sounds to me like Jet2/easyJet/Ryanair may be pitching BOH & SOU against each other in a bid to lower costs for a new base or sizeable expansion. 1m pax will not come from a couple of w flights inbound from Europe.
Or
TUI could be considering basing a single 737-800 there (that would only be 250-300,000 pax though).
Others have said BA Cityflyer would be a great fit for SOU. Assuming for a moment BA Cityflyer based all their new 4 aircraft orders at SOU instead of LCY that would only account for 750,000 pax.
And...
Have Flybe pledged to upgrade their Q400's from 78 seats to 90 seats? This could also be part of the forecast.