as long as the Design Gross Weight is above 3500kg it is mot & tax exempt as it falls into a
different mot class which has an exemption for "working" fire engines.
the DGW for a standard series fire engine is unlikely to get that high so they would only be tax exempt, but you will be ok with a forward control (don't know about the rangy 6 wheelers)
Not sure about the definition of a "working" fire engine but the only time i've heard of police intervening was when they found someone driving around in an ex fire service transit van and was still claiming the tax exemption, so i think you should be ok with a real fire engine.
p.s. welcome to the wonderful world of fire engines, you can bring your engine along to any show in the tattiest of states and still people will look at more than all those other boring classic vehicles, just keep an eye out for the kids who think they have an automatic right to climb onto the roof just cos it has a ladder