Pug is right - why do I fly from Heathrow? Choice, price, frequent flyer program, you name it.
I am quite surprised that anyone is taking this seriously - clearly, airline economics have passed them by. The whole point of a hub is that enough people will use it to create enough traffic to make it work, both in terms of terminal traffic (which is going to be hard, since there really is not that much demand for most of these routes in the South West of England, South Wales) and in terms of interline traffic.
You don't just 'create' a hub - you need to get a very major player - a VERY major player - to come and cornerstone it, or alternatively get a whole host of network carriers in there.
Say they get TAM for South America, Etihad for the Middle/Far East, KLM is already there for Europe (although if they lose all their connecting traffic they may not be), and say Kenya Airways for all points south. Is that enough? No, no where near enough. Close down Bristol, Gloucester? That would help, but do you imagine that that would be possible, even with a barrage? Not a cat in hell's chance.
Pie in the sky, I'm afraid.
TA