BE and KL absolutely did not coexist at SOU. It was a mess for both and KL withdrew before returning only after BE Mk1 had departed this life.
And I didn’t think the Aer Lingus or BA brands were too shabby ?
None of the core routes at SOU are strong enough to take two carriers in competition. All are good single carrier routes and poor two-airline routes - they’re just not thick enough to warrant two airlines. The airport has never sustained it and nor will it either.
The ongoing fantasy of easyJet at SOU is clearly too much for some to resist. The runway extension will help current carriers who are payload restricted even on an ATR72 some days, let alone an E145. It will also help with some more leisure flying and there are two options - neither easyJet - that I can think of which would work well to deliver it.