"Residents with the equidistant geography between SOU or LHR would naturally flow to LHR."
Don't agree either. I live in West Berks, about 40 miles from LHR and SOU. Back a few years ago when I was flying for work trips I always used SOU to go to places like JER, GCI, BHD and GLA. LHR (or LGW) for longer-haul stuff.
My SOU trips were nearly always daytrips aswell. SOU made that easy as, for example, at LHR it's just not possible to be driving out of the airport 10 minutes after getting off the aeroplane. I lament that many of these daytrip options now seem to be increasingly unavailable.