A very large portion of passengers at LHR are in transit - the only contribution they make is at WH Smith's
By golly, I think you've cracked it.
If all arriving passengers were frog-marched to the Tube or coach station to ensure that they couldn't board another departing flight, that would solve the capacity problem at a stroke, because many routes would no longer be viable.
Now why hasn't the Airports Commission thought of that?