I believe SS has summed up the UK well. Our infrastructure stinks! Heathrow is a pathetic excuse for an airport. It's in the wrong place, poorly connected and incapable of being expanded. An additional runway will do nothing to improve efficiency nor will it add any worthwhile capacity.
Why? I'll explain as simply as I can. And it is simple. When people travel, they go from where they don't want to be to somewhere where they want to be. They measure their success in price, hassle and time. Trevelling pleasure is no longer expected because has been removed by overcrowding, standardisation, oppressive security and the sheer greed of the thieves that run airports.
[The public don't expect to take three hours by road to cover 45 miles, spend an hour parking, pay £80/day for the privilege, another hour getting to check-in and then after passing the Gestapo think "Oh my! Look at that lovely baggage, I must buy some." Nor are they amazed by the price of the cameras or the standard of catering. But they are never surprised when there is a delay. But rather incredibly, there are people who believe that LHR is a worthwhile place.]
Returning to where we started: You might as well place London's third runway in Belgium for all the good it will do. The only realistic thing London can do is build a new airport somewhere new, build it quick and then bulldoze Heathrow. Gatwick, Luton, Stansted will remain as the airport equivalents of clip joints; squeezing as much cash out of point-to-point leisure travellers as they can.
My job depends on Heathrow not being viable as a UK hub. But I'm not worried. I'm convinced I'll keep my job because of our worthless politicians, greedy bankers, NIMBYs and a travelling public who are prepared to accept sub-standard airports. This public convenience will provide me with employment for years.