Speke said: "The normal attendance of the 'outside fire service' (and other services - ambulance/police etc) at an Airport emergency, is surely essentially the same as any other call out. It is done based on availability and priority. "
I don't know precisely what is involved or how the turn-outs are arranged but from experience I would say that there is definitely a pre-determined attendance and not with just a few odd vehicles showing up. The mass of fire vehicles and ambulances which arrive at the rendezvous point within minutes of an emergency being declared suggest this - and I've watched it hundreds of times at Heathrow.