The fire engine guys are going through the mill with this at the moment because there is a lot of debate about what constitutes a load. Water and tools are necessary for fire fighting but constitute load when on a preserved vehicle since they are not fixed to it and are not necessary for the function of the vehicle (ie. fuel, oil, personal toolbox etc.) I think it's ridiculous because there are so few of them out there that it hardly seems worth the bureacratic effort.
Point is, I'd suggest you don't focus on it being a fire engine. Think of it as an old lorry. Therefore, it's over 3.5t, was registered pre 1st Jan. 1960, is not being used laden or towing a laden trailer (insert your interpretation of 'laden' here ;-)), is not being used for hire & reward AND you received your license before 1997, you can drive it on a car license.
- Mike