The feasibility for the Thames airport in London has been discussed already on these forums, however whilst it has been recognised that additional capacity will be needed, I haven't seen much mention of other potential sites. I'm not dismissing Thames as a poor option, but it seems like other options haven't been discussed to the same length. (Obviously I'm willing to be proved wrong, but I have used the search function...
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I understand that realistically any potential site should to be in the southern regions of the UK, to be able to satisfactorily relieve congestion at Heathrow/Gatwick, and support Stansted/Luton.
For example, there seems to be a fairly open stretch between Biggin Hill and Rochester. To me, that would be able to integrate well into the traffic flows of City, Gatwick and Heathrow, better than the proposed Thames airport. Not to mention the advantageous position to HS1.
I know this is a very simple way of thinking at the most basic level, however this idea is firstly there to be shot down, secondly to promote some more ideas.