Any police officer who exceeds the speed limit is breaking the law because the law applies to everyone. However, that's not the same as saying that he will (or should) be prosecuted for doing so.
The Crown Prosecution Service exercises discretion in all genuine emergencies. That not only includes the emergency services but also, for example, a parent rushing a seriously ill child to hospital.
The CPS produces staff guidelines to ensure that the same standards apply throughout the country. Unlike many 'official' bodies, their internal guidelines aren't kept secret from the public. Anyone can view them, here:
http://www.cps.gov.uk/legal/section9/chapter_b .html#08
Chris