Unmanned transport
You mention some 40 carriers which could apparently "off-load" their passengers into Continental Europe instead of LHR. I am sure this is possible, but the question mentioned several times above is why would they do that?
Essentially airlines are no different to any other business - they provide supply to meet demand. Therefore it would seem more people want to travel to London than some other cities in Europe.
This is a reflection on the realities of economics - nothing more than that. Given the pressure on all airlines these days it is very unlikely they fly to London instead of CDG/FRA beacuase they want to - simply because thats where the demand is and hence where they may make money!