So what happens to those aircraft who have creatively "committed to Heathrow" due to a shortage of fuel (on the basis that Heathrow has two runways and the weather is reasonable) when both runways suddenly shut?
Do they land at Heathrow without fire cover or do they now demand priority over everyone else who is diverting at the same time?
That's what the Pre-Accident Plan sets into motion - ATC knows the given airport is closed to traffic and executes the established plan to divert flights to nearby (and there are several in the London area) airports. It's the controllers' job to sort out fuel issues. If a pilot failed to adequately plan his fuel, or if winds were greater than forecast, ATC will assist as necessary. In fact, ATC will begin diverting aircraft long before they are "committed" to make room for those that are already committed.
Again, there are sentient life forms in abundance beyond the wing tips, no less the cockpit.