LHR Fire cover
I've seen fire cover requirements refering to ICAO Annex 14 mentioned here.
The actual place to go is Chapter 9.
There you will find the level of fire cover required based on size of aircraft.
This is where Heathrows problem is somewhat obvious.
The amount of water/foam necessary to be available for Cat 6 and above aircraft, which most of LHR's movements are these days (i.e. Widebody's) is about 9 times the requirement for Cat 4/5 typically smaller 737/A319.
So, LHR probably could have stayed open for the smaller aircraft and remained legal for CAA purposes but couldn't meet the higher level required by most of its movements. I can see the headlines now "Newcastle A319 allowed to land while Beiging flight forced to divert", probably easier on ATC to divert everyone than pick and mix.