I was told by the copper you can be done for 30.1 in a 30mph zone as the tolerance system went in the year 2000.
One of three scenarios are likely to be true here:
A) That conversation never happened.
B) The copper didn't know what he was talking about.
C) The copper was just trying to illustrate to you that you can *potentially* be "done" at any speed over the limit - which you have taken too literally.
The Unipar SL700, which is a very common laser device, is only accurate to +/-1mph by it's own manufacturers spec sheet, so the chance of CPS
ever prosecuting, is zero %. If in some strange imaginary world, they did decide to prosecute - it would be thrown out of court in approximately 0.1454143 seconds. If the laser device can potentially be out by 1mph, that means 31mph would never be prosecuted. If you aren't on the same level as the car (ie, if you are on a bridge or slip road), this can cause an error margin also. This is why there is a margin of error allowed before prosecution, because it would be impossible to proof 31-32mph in court as there are too many reasons why that speed may be inaccurate.
CPS have a threshold that they wouldn't run a prosecution at, up to and including 33mph IIRC (34mph being the first they would).
It's very similar in concept to drink driving - 36microgrammes/100ml of breath is illegal, but CPS will not charge until you hit 40microgrammes/100ml, even though the law says 35microgrammes/100ml is the limit. This is to allow an error margin and to give only safe convictions.