I finally managed to find the text of the Road Safety Act 2006, which has expanded the exemptions from observing the speed limits. However, it does not appear to have changed the definition of an ambulance. The Secretary of State can now prescribe, by regulations, other purposes (in addition to those relating to fire and rescue authority purposes or for or in connection with the exercise of any function of a relevant authority as defined in section 6 of the Fire (Scotland) Act 2005, for ambulance purposes or for police or Serious Organised Crime Agency purposes) for which vehicles may be exempt from speed limits. The new section provides that the exemption from speed limits does not apply unless the vehicle is being driven by a person who has satisfactorily completed a course of training in the driving of vehicles at high speed provided in accordance with regulations under the new section, or is driving the vehicle as part of such a course. FRUs and MRUs may now be prescribed vehicles, but I can't find any regulations to that effect. It wouldn't surprise me if they hadn't got around to it. And don't rely on the police not prosecuting. In the case I was thinking of, the driver was charged by Lincolnshire police but not by Cambridgeshire. The case was eventually dropped by the CPS.