First of all, for the sake of argument I support a third runway at LHR, as well as a second usable runway at LGW. No airport should be prevented from expanding and thus becoming inefficient and problematic for passengers.
I do balk at one argument made by the LHR side over routes though - over routes, both domestically and internationally. There could be up to another 240,000 annual movements, or around 650 movements per day. In the initial phase, I'd expect only about half these to be taken up, before the runway fills up again. For every 10 slots assigned, at least 2 needs to be assigned to a domestic route and 1 assigned to a route not already operated by someone else.
1) Only two airlines in the UK have the capability in their current form to operate routes to additional domestic airports - these routes need to be operated by smaller E-jet, ATR or C-series equipment. Only BA Cityflier and FlyBE have the fleet, unless there is a new entry, to operate these routes profitably and efficiently.
2) If these routes were profitable, wouldn't BA (or another airline) be operating them already?
3) Because of point 2) there would need to be government / airport guarantees (can't say I'm a fan of this approach) for certain % of slots being used for certain routes, otherwise the usual suspects will just be doubling down on typical dense routes. Heathrow have stated that Dundee, Prestwick, Carlisle, Norwich, Durham/Tees, Humberside, Doncaster/Sheffield, Liverpool, Derry, Isle of Man, Newquay, Jersey, Guernsey could be operated in addition to those domestic routes currently operated.
4) Internationally, many routes that LHR have highlighted as potential new markets surely are limited and would prove extremely costly to operate:
From west to east:
United States - Portland, San Antonio potentially limited J traffic and easy connections through existing hubs? Certainly think Orlando and Memphis could work; the former already operated from LGW.
Central America - Guatemala City, San Jose, Panama City - think these are definite growth areas by a time a runway is built, but I think limited to a 787 (or going via MAD).
South America - Quito, Lima - certainly think these would work. Caracas, Belo Hortizonte, Porto Alegre - sceptical about these; the latter two could work if done on the same trip.
Africa - Think Dakar, Entebbe, Dar es Salaam, Durban and Khartoum could work. BA have tried Monrovia previously and pulled out - no suggestion that this is a growth area. Mombasa did have a huge charter presence which seems to have tailed off due political instability. Extremely sceptical about Port Harcourt, Lilongwe and Harare. These routes are bread and butter though against the ME3 who will easily beat everyone on price.
Middle East - Baghdad not exactly going to be a big business/tourist destination anytime soon. Damman high J potentially.
South Asia - Peshawar is one of the most dangerous places in the world, so that's a no from me. Think Goa, Kochi, Thiruvanathapuram, Kathmandu, Kolkata could all work, albeit with lower yields. Think India will become a huge aviation powerhouse by 2030. Having worked over there for 2 months last year it was staggering the increase in human development and just the shear scale. The airlines over there are putting in huge aircraft orders. Again though, bread and butter for the ME3.
China - Wuhan, Chongqing, Nanjing, Fuzhou - probably just scratching the surface for that country.
South East Asia - Penang, Denpasar - I'd say Indonesian aviation is about 10 years behind India, but the likes of Lion are making huge strides in that region.
Far East - Surprised Osaka isn't being operated already.
Oceania - Brisbane is the obvious one, but will depend on "Project Sunrise" - the economics don't really work on Perth...