Hello Shed!
Are you familiar with Bastiat's broken window fallacy?. Spending money on projects which make no financial sense is never a good idea however you spin it. Consider also the concept of opportunity cost. Whilst you waste funds on a project of low merit, those funds cannot be simultaneously be put to productive use elsewhere.
You seem to be under the impression that the government is footing the bill for this project. Investment is in place based on the sound business case of LHR expansion. I am sure you and a few others know better as you obviously have better business acumen and access to information than those more than wiling to put up the £bns to fund it.
Bastiat's fallacy does not come into play here as this money is coming from outside investment. You will also notice, Shed that the surface access works are set to be priced accordingly now.
So we have £bn's in outside investment coming into the country and the nations tax coffers in return for a public spend on infrastructure that is in need of upgrading anyway. That investment is then spread around the whole of the UK and we end with a world class hub airport.
As people are now transiting through LHR instead of AMS etc. we have further future income into the UK which you seem pleased to give away.
The above can also be said about any other project anywhere in the UK. There seems to be enough investment available from China and the Middle East. I would suggest investment opportunities come with a better business case when based in the SE and that is why funding is more forthcoming. These projects should not be cancelled out of sour grapes when the construction alone would be of so much benefit to the regions.
As this can be built now and easily linked in with Crossrail and HS2 it really is a once in a lifetime chance to get it right.