Torquelink
This means that one of two scenarios are true:-
Either, they have got the supply and demand right, and that they can therefore charge an economic fare on SOU-PMI, which permits them to make a handsome profit from passengers who will pay for the convenience of flying local to driving to LGW.
Or, most of the seats are blocked off for tour operators making the service a quasi IT with very few seats on the open market, driving up the price on the few available..
You can make the same comparisons, for example, flying to Newark on Continental from the "regions" (that's anywhere outside the "favoured southest") and London. Regional fares are never as competitive as London ones - I suppose it makes up for their high cost of living!