Boris wanted a new airport in the Thames estuary but that died.The cost, both financial and Eco, (political) would be too much.
I live near a costal airport, (Bournemouth), and don't mind the dozen or so flights a day but a half million others in the area may not like lots more.
If you mean right on the coast then all those holiday beaches could be covered in tarmac for runways.
I think we have to stick with the system we have, but why HS2 isn't going via Heathrow I don't understand and also connecting with HS1?
Dixi.