A vehicle is considered to be speeding if it travels at excessive speed for the prevailing conditions, or above the posted speed limit.
You're guilty of speeding once you travel at 1 mph over the posted speed limit, and for that reason the police refer to it as an "absolute offence". If a police officer tells you that they won't pull you over on a motorway at under 85 mph that's either their personal opinion or the policy of their constabulary. This is why they won't need any changes in legislation to introduce "zero tolerance".
Operating at a speed, possibly below the posted speed limit, above that which a reasonable and prudent person would operate under the circumstances.