I'm confused. Does a hub need two runways or three ?
Any really busy airport requires more than one runway.
When it gets really, really busy (like LHR is) it needs more than one runway that can be used for landing at the same time, i.e. 'independent simultaneous instrument approaches'. To meet the requirements for 'independent simultaneous instrument approaches' the runways have to be far enough apart. At LHR the runways are not far enough apart (there is no airport in Britain that has runways far enough apart). When a third runway is built, the 'outside' two runways
will be far enough apart.
(LHR is running at about 98% capacity. At busy periods the spacing between landing aeroplanes is about 2 miles. In 'low visibility' conditions the minimum spacing safety requirement for most aeroplanes is about 4 miles, so when it becomes foggy about 48% of capacity is lost, hence all the delays and cancellations every time the fog rolls in there.)
Three runways are useful as two can be used for departures and one for arrivals at 'departure peaks' and two can be used for arrivals and one for departures at 'arrival peaks'. This ensures more efficient and therefore safer operations (although safety at LHR, together with probably most British airports, is very good anyway)
AND it helps to reduce concentrations of noise by reducing the concentration of traffic on departure and arrival paths during those peaks.
Despite everything that any 'green' or pseudo-'green' politician might sprout, that extra runway at LHR is a requirement if there is not another British 'hub'
instantly available. Failure to get on with that runway fast will have the hubs at Amsterdam, Paris, Frankfurt, Madrid (
all of which have more than two runways
and sufficiently separated for 'independent simultaneous instrument approaches'), Doha and Dubai (and probably several more too) rubbing their hands with glee at the stupidity of British politicians.
It is far too late to be thinking of a 'new airport'. It will take too long to be ready to use (just look at how long the 'efficient' Germans have been with getting Berlin Brandenburg ready!!!!).
I'd favour a Gatwick runway
It is the wrong side of that 'traffic jam' called London to be a properly 'British' airport. It is a 'London regional'.