Also, re: transfer pax.
I think their benefits are over-stated. They aren't spending money outside the airport unless there is a major delay. I have done a same day transfer (ATL), and headed downtown for a bit of sightseeing. IIRC, I spend about $3 on metro fares and $8 in a fast food joint. Visited the MLK memorial, which was free. Maybe airports themselves prefer the transfer crowd - usually a little more dwell time, and maybe an overnight? Would love to see the stats of who actually stays in airport hotels. I know airlines usually charge for 2 sets of PSC, but what are they actually paying? If they also don't have to be screened, then it starts to look juicy - except that there's one thing airports love more than anything else, and transfer pax never use them - cars! No parking charges, no hire cars to pick up, no cab fees.
Surely what we want is o&d, and really more d than o, as o sucks money out of UK plc. Forget the willy waving about who's airport is biggest - let Beijing take that crown from Atlanta. So what - Atlanta is nowhere on the world cities list, and it never will be. The world's most 'useful' airport - ie with the most O & D - not transfers? Somewhere near you perhaps SS?
I've never seen a figure on which airport actually brings in the most visitors (outright, not as a %age - saying 95% of passengers at LDE are inbound in meaningless). One figure is that for every £1 tourists spend in the UK, £3 is spent abroad. I think the scales are more balanced in London.
Of course I don't want to put people off from visiting. The govt are perfectly good at that as it is - and the fact they are even looking at this is totally inconsistend compared with their no runways in SE & APD policies - almost to the point of being bizarre, or as if no-one can resist the temptations of a megaproject.
So the French can keep their transfer passengers, and we can just muddle by letting LGW fill up, then STN, then LTN and so on. Sexy it ain't, reality it is.