Seem to recall a time not too many years ago when Bath Travel, later Palmair, contracted Princess Air with a 146 to fly Sunshine routes out of SOU and even that aircraft had problems in 'hot' weather taking off with a full fuel load and full complement of pax... I can recall at least one occasions when the aircraft departed with 50% fuel and 50% pax, flew down to BOH where it boarded the rest of the pax who had been bussed down, and topped off the fuel...
Also seem to recall that that happened a couple of time last year with the Embraers on the Spain routes...
Southampton was, is and always will be limited by its runway length.
BOH will always be limited by the narrow mindedness of the various councils in the area and the airlines... It's a Catch 22 situation the councils will not improve the road / rail transportation links until it is justified by an increase in air traffic... and the airlines will not increase their routes and services until the transportation links are improved...
Getting back toSouthampton though... why is there no direct public transport link from Southampton Airport to Portsmouth... currently the only direct route is private car or taxi... no buses, no trains, Nada...