Being a Londoner I have every idea about how screwed up the air tranpsort system is.
One of the first problems is the basic fact that the airports are all privately operated to make money, not provide a service. BAA want to make money, not make life easier (T5 will/would be a cash-cow)
You cannot grow Heathrow. Full stop. It is over-capacity, and bar razing it to the ground and moving out the neighbours, changes are limited. Forget it.
Stansted still has a but of room; but bare in mind it is at minimum 1h 20m from West London (where most of the rich people live) and 1h from the City. It could not be the primary focus for London, and it looks like no intercontinental services can get going. But its a nice airport
Luton is being redeveloped, since it is currently a hole, even further than Stansted. Gatwick is actually in a near identical situation as Heathrow; London City is less than 1% of London traffic; so it seems we return to the point of regional replacement services.
Manchester is growing into a major international airport. No problem there. But in my opinion the UK can only sustain one large airport outside London. It is unrealistic for those in Bristol, Birmingham or Leeds to have a major airport when our country is so small.
The answer is for the REST of the transport system to be improved - so people don't have to fly to Heathrow. They can go by public transport. It would be much easier and more beneficial on the whole to invest in an efficient train system that connects our airports, like they are trying to do in France with CDG.
We still need an airport like CDG...but thats just a few ideas up in the air.
P.S. I am sorry but I think the stat. about population between Mersey and Humber blah, blah, blah is false.
I think you meant within a 70 mile circle, or 35 mile radius. Which means you would get everything up to the greenbelt - but theres a lot more just past it. And you forget that it is much quicker to travel around the SE than the NW. Middlesborough to Liverpool takes much longer than London - Bristol...not to mention the difference in business travellers...anyway...i was just surprised