Agreed that Stansted is suitable, however the airlines appear suffer in Britain of Heathrow syndrome, where all the national flag carriers and other full service airlines all wish to fly to Heathrow. Gatwick now hosts easyJet mainly, BA to a lesser extent, some Eastern European flag carriers, a small Ryanair operation and of course all the package tour airlines.
They appear to have a similar situation in Paris where many major airlines will only fly to Charles de Gaulle, even though Orly is rather well positioned. Quite remarkable.
Anyway what I'm trying to say is that it will be a struggle to pull airlines away from Heathrow, but it is something that will have to be done in the near future.
If you can do it at Manston then all the better - because in 50 years time with a shortage of fossil fuels I should doubt very much that we will need asphalt runways in their present form and therefore concreteing up the Thames is demonstrating a considerable lack of foresight.