That is the national equivalent of trickle-down economics - the argument used by the wealthy to justify low taxes for them. This is generally recognised as garbage.
I didn't use the phrase 'Trickledown' as it tends to be associated with the economics of the finished project. I was specifically talking about the construction phase and the large firgures associated with the construction.
Go to any large construction site in London and you will hear regional accents from all over the UK. Have a look at the addresses on the sides of the lorries bringing in the supplies and machinery and you will see that hardly any of it comes from inside the M25.
Anyone that has travelled up the M1 on a Friday evening knows how many tradespeople there are going home, with a large pay packet in their pocket, to regions all over the country. This money is then spent in the local economies and benefits others not working one the project itself.
To think the money spent on Heathrow expansion or any other London based project is only staying within the M25 is ludicrous.