I don't see how "short-haul demand to established bucket-and-spade favourites" helps the UK economy or grows jobs across all of the UK.
Well, you are entitled to harbour a personal dislike of the leisure travel sector if you wish but the market exists in volume regardless of your disdain. Companies such as TUI, Thomas Cook, EasyJet and Ryanair are major employers and support further employment across the wider economy. Leisure travel encompasses inbound traffic as well as 'Brits abroad'. The hospitality and tourism industries support substantial employment in the UK. These are labour-intensive sectors.
The flights associated with these sectors require runway capacity too. Quite alot of it actually. To extend your logic, London requires no new runway. In fact, you could close afew. Just ban foreign holidays and there will be plenty of airport capacity for the 'suits' to fly wherever they need. But this is the real world, and the healthy, growing leisure travel market is a major part of the demand equation. Demand which an expanded LGW is well-placed to address on behalf of the SE.