LTNman
As mentioned before, whilst the extension will open up more leisure options, it will also greatly help existing operations become more operable. I used the Munich flight when BM operated it a number of times and some of the seats had to remain empty to operate it. The 50-150 seat regional jets would be used on business routes and they currently cannot operate at full potential.
As for leisure, do you think such flights are only about Brits going to the south of Spain to sunbath for a week? I know that’s some airports market but the catchment here is rich and potentially has more sophisticated tastes. City breaks can be served which will also cater for business demand, Berlin for example which BA are operating. Also as mentioned above, the south of England is a holiday destination for continental Europe so it is inward holiday passengers as well. Not to mention continental Europe and anyone else internationally that wants to cruise Europe needs to come here to start their cruise and leave here when it ends.
As things stand the prices for flights at SOU are large as a result of the operational limitations, it’s a whole different story when prices can come down by at least 50%.
So to answer your question the extension is not just about bigger aircraft, it’s existing operations. Plus there are lots of routes where aircraft could serve both leisure and business needs.