I seem to remember the exemptions for FRS drivers were as follows:
Exceed the road speed limit only where safe to do so without causing a hazard
Treat a red traffic light as a give-way junction
Negotiate one way streets in contraflow to the designated route
Sound sirens and horns between the hours of 2300 & 0700
Sound sirens & horns when stationary
Park on common ground
Leave the vehicle unattended with the engine running; (referred to by Yesterday)
There may be more but I'm running out of available memory space on my cranium hard drive, however, I do recall that even following the guidelines, (and thats all they were), of the first 3 above, if any driver of an appliance, (irrespective of urgency or advice from his OIC), who causes an accident is still subject to prosecution