If STN or LGW build a second runway, it won't be used to capacity IMO, whereas if LHR gets #3, it'll fill up rather quickly. The stats LN-KL has kindly provided just show how good London ATC are at getting good runway utilisation, especially considering the antiquity of some of the software/hardware at Swanwick (because of their levels of traffic, they simply can't update as often as the rest of us).
I appreciate some of contributors here have their own stakes, IE living in the noise corridor, or working at another airport, or just loving LGW, MAN etc, and I also appreciate as a non U.K. Resident, my taxes won't be being used to pay the bills, but the only way the U.K. can continue having a world class airport is by building a third parallel runway at LHR.
If this does not happen, AMS and DXB will quickly relegate LHR to the second tier in terms of global airports. The benefits of having a genuine global hub cannot be overstated. The exchequer gets massive money from direct taxation of transfer pax. More shop workers/pilots/cabin crew/controllers/cleaners/mechanics/engineers/electricians/plumbers/paramedics/fire crew and all the other support staff are employed as a result, and they pay tax. Combining these transfer pax with point to point pax results in new routes that couldn't survive as purely point to point. These new routes allow direct links to businesses, meaning, for example, companies in places like Lima, Brasilia, various other South American and Chinese cities amongst others without current direct links can establish partnerships or European offices of their own in the U.K., employing more people, who pay more tax.
A split hub system really will not work. Let's say you are a businessman based near a relatively minor airport anywhere in Europe. Are you really going to fly into LHR/LGW and get the bus to the other because each caters mainly for different parts of the globe, or are you going to jump on the likely direct EK/Q R/Unmentionable airline service east or UA/DL/AA service west and connect through their respective hubs? The same goes for companies sending or receiving air freight.
The U.K. lost it's aircraft manufacturing industry because of the "That'll do" attitude. Putting 3 hairdryers on the Trident and allowing the 727 a free run is an example of this. Your automotive industry, barring tiny companies like Caterham or Ariel is all foreign owned because of a similar attitude for years, Jaguar and Land Rover are Indian, Lotus is Malaysian, Vauxhal is a GM owned company selling rebadged Opels, Bentley and Rolls are German. Don't let your once world class airport die because of the same thing.