I for one would agree with the above sentiment re. LHR. I live in W Berks (almost exactly equidistant between LHR and SOU) and will always use SOU for domestic and local Europe (where available) flights.
The fact that I can turn up at SOU, park outside the Terminal and be back in the car on the way home 10 mins after landing are always going to trump the LHR experience for short flights. In my time I've done GLA, EDI, BHD, BRU, FRA as well as JER and GCI from SOU and it's a pity some of those are no longer available/can't be made to work.