A radical, cheap, quick option that's probably totally infeasible
Just a suggestion from someone who has no aviation experience except as passenger, during which I've noticed how much runway is unused even when I'm in a long-haul widebody. Couldn't the northern runway at LHR be extended a bit and made to function as two shorter runways? It looks to me as though it could then handle twice as many small- and medium-sized aircraft, which account for a surprisingly (to me) large number of movements at LHR. At peak times for jumbos it could function, as now, as one longer runway. I am sure it's unprecedented, and in any case wouldn't be ideal, but it seems to me much the cheapest and least disruptive option, and one which could be implemented much more quickly than any other. It's probably unworkable, but I'd like to know why before withdrawing gracefully from this discussion.