At first I thought it was April Fool's day!
I have thought that a solution to New York's airspace problem would be the linling if JFK & LGA but that is the ability to consolidate masses of flights by commuter jets. You don't get many of these at LHR & LGA.
Does the Government envisage a major shift in the type of traffic at the airports? LGW is largely loco & charter, LHR legacy flights to business destinations. Sure a few pax are transferring from Jersey to long haul and domestic to MCO / TPA / Caribbean but enough to justify a £5bn scheme?
Lets say the capital & operating costs are £500m p.a and 10m pax p.a. (rather optimistic) Thats £50 per trip - about the same in per km as the Heathrow Express. I can't help feeling that KLM could charge less for an itinerary via Schiphol which might reduce the take up.
The trouble with this Government is that on the one hand it seems to be seduced by 300 km/h+ railways whilst doing nothing to integrate the transport network. I suggest that BA gives some ministers / advisors / civil servants a free open jaw ticket to ZRH & back from GVA or vice versa so that they can see how to effectively spend money on rail upgrades.
If they want to build a new railway, route it via London as that it will benefit other users. There is no reason why you can't have a sealed coach on a train used by local passengers - Eurostar sometimes clears customs at St Pancras.
If capacity is constrained logically short haul to long haul transfer traffic will reduce and with it pax wishing to transfer between the two airports.
Here are my thoughts on increasing capacity.
* Limit domestic routes to one airline but force the operator to sell space to competitors. (That would require a waiver from the EU.)
* Encourage routes from regional airports. Possibly VS could make a go of more routes from MAN but realistically it will be foreign carriers such as KL, EK ou CO/UA. (To me the argument for the 3rd runway at LHR isn't about lack of air capacity for the UK but about lack of opportunities for UK airlines / workforce.)
* Build an link from the Midland Main Line to LTN. There is currently a bus link but to be competitive you need a station directly under the air terminal.
* If you are going to build a link between airports rebuild Northolt's runway so that it is parrallel to the two at LHR, build a terminal on Ruislip Underground depot (which will be rebuilt underground like that at White City) and link it to the centre of Heathrow. OK its probably a non-starter owing to local protests (although it might be worth examining how a runway was built at Silvertown (LCY) with minimal fuss).
Of course we have a rather underused high speed line in this country out of St Pancras & "Borisport" could be served by a branch from it. Indeed it would make the £2bn paid for the HS1 concession look rather good value. Perhaps it would make sense for an airport and high speed line to go together.