I've mentioned this elsewhere, but still don't know the answer...
I can only speak for Heathrow but it probably applies at many other places - there are 5 categories of emergency. For the lowest the AFS turns out. For the next 4 the local AFS is backed up by the outside Fire Brigade and other services which head for ATC-determined Rendezvous Points around the airfield. The lowest category for which this procedure applies is a "Full Emergency", which will be declared by ATC for a loss of 50% power, undercarriage problem, any hydraulic problem, etc.
Question: If the outside Fire Service is not available what happens?