This is such a simple problem to fix seeing as we have three airports around London, it is a joke why the civil servants and government have not already thought of it instead of shouting about building a third runway at Heathrow.
You make Heathrow the main centre for long distance flights covering America, Far East and those types of distances. Gatwick becomes the main airport for europe, middle east and similar distances and Stanstead becomes an over spill and also covering internal UK flights and ones to places like Paris and cities within that type of distance.
The last thing screw one airport being a hub, all three become the hub.
The one problem is the links between all three, that will need working on, but the airport problems are fixed though.