Agree with a lot of what you say Flitefone, but not in the case of the useability of UK (and a few close Continental) trunk routes from SOU, with reference to your "nine in ten times they don't" comment.
The choice/frequency from SOU to places like EDI, GLA, MAN, BHD, DUB and AMS is pretty good, and personally I would always choose to fly from SOU to these places rather than have to do the same through LHR or LGW.
It's just easier/less time-consuming due to proximity of car parking, trains, less distance to walk to through the Airport etc. at SOU.
I say the above as someone who lives slightly closer to LHR than I do to SOU.