That smokin' hole is getting ever closer.
I agree; one day the unexpected weather en-route, combined with an incident and lack of contingency fuel will catch out not just the odd flight but, many at the same time all trying to get into LHR, LGW and STN.
Multiple fuel priority maydays and even if all the current runways are available and take-offs cancelled there would be chaos. If the holes in the swiss cheese really started lining up and another incident say LGW runway blocked by a disabled aircraft then someone may not make it! I believe it takes at least 20 minutes to get LGW's emergency runway available.
So where are ATC going to send you - Biggin Hill, Farnborough, Dunsfold, Lasham, Bournemouth, Manston, Southend - Possible lack of Fire Cover at these airfields but you may be lucky and get it down OK.
The lack of runway capacity is a safety factor that should be addressed by the CAA and Government forthwith. In my view it's unsafe that LGW and STN have only one runway for the traffic they're taking and LHR should have at least three now!
I'm sure all the NIMBY noise protesters would prefer the extra runways to getting the out of fuel aircraft instead.