New runways don't happen overnight, so there is still a need for somewhere from which additional services can operate in the immediate future. Various non-UK airlines seem to be building profitable operations from outside SE England, going both east and west. If the UK industry is not to lose more market share, it needs to look beyond the London area.
On the other hand, London-Calais would be as accessible as Boris Island, not have the birds and bombs, and would even avoid APD.