Hooligan Bill you are correct in what you are saying. I've done a spot of research and it seems that most of the larger airports all have contingency plans for when this strike does take place.
These range from having greater water reserves to having a few extra appliances on site and available for use. This should mean that for the majority of airports (certainly for the passenger ones), there should be no downgrades in fire cat needed.
From what I understand there only becomes a problem if something goes wrong with any of the appliances, then a possible downgrade may be required, but that is what would happen even with external fire services available.