Well this support for LHR from other cities is very forward thinking; in fact in advance of anything seen before on the UK Airports scene.
So let's intoduce a few boring practicalities. First of all any advantages for them are very unlikely to be realised until LHR gets its additional runway capacity available.
Although LHR seem to be dangling the carrot of lower domestic charges from next year, who is going to take advantage of it when there will be no new slots? Airlines will therefore want to maximise the value of their slots and I don't think there will be any flights to LHR from unserved domestic destinations next year somehow.
When will new runway capacity become available? That's a big ask bearing in mind that firstly Davies has got to decide that LHR is the right place - quite likely on the basis that 2 chances out of 3 are at LHR ..and this assumes he makes a decision at all. But Davies having made a decision, what happens next?
The incoming Government (whoever he she or it may be) has to make a decision about what to do. I suspect it won't be a high priority bearing in mind all the other things that need fixing. Anything too controversial such as this will no doubt be parked on a taxiway somewhere for a while.
So assuming someone finally makes a decision to put another runway at LHR what happens then? There is the planning process to go through whatever that is this week as the rules seem to change every couple of years. And the possibility of endless legal challenges, the only winners of which are m'learned friends
So permission is finally granted, the runway is built and new slots are created? Who gets first dibs at them? Under the EU slot regs, "new carriers" at LHR (basically those with very few per day) have 50% ringfenced for them. The remaining new slots go into the pool. But you can't ring fence slots for domestic routes unless they are seen as Public Service Obligation route which have to be approved by the Government under EU set criteria. And current UK Government policy is very strict.
However, that last paragraph may be nullified if the UK exits the EU
. But there again the UK government may chose to leave some non- controversial EU laws such as this on the statute book; if not maybe new legislation will have to be enacted?
The upshot of all this is that I think it will be a very, very long time before any new domestic routes into LHR will happen. In the meanwhile the world will go on and if airlines wish to continue to develop in the UK market, they will look at opportunities outside London.
If the cities that are supporting LHR expansion are expecting something to happen in the near future, I think they will be disappointed. So they ought to be also looking at supporting other airports in their regions that can support flights to new long haul destinations in the short term as well as backing LHR for the longer term.