John,
I believe the rule is still that one must obey a legal order.Though it is true that some orders may only be true in certain circumstances. I cannot, however, see how an order from the government of the day to cross a picket line could be considered illegal, which is the case in point. if you wish past precedent, they did it in the General Strike of 1926 when the forces crossed the picket lines to run the buses and the underground.
You may quibble as to the ownership of the fire engines (government or local authority), but that is a separate argument and one presumes that, since they are the people in dispute with the firemen, they would authorise their removal and use.