It's all about discretion though isn't it, and for the most part Police do allow for this - however some will not. A prime example and abuse of this, is Section 59; first legislated to cover the boy racer problem and cruising, burn outs in car parks etc. However, Police are now using this for minor offences where otherwise there is no proof.
Not only that, and this is where it gets interesting, a member of the public could report you, for let's say a noisy intake (Forge twintake, Neuspeed whatever) or what they think is you speeding, a Police officer in uniform could then come round and issue you with a Section 59, something that you have Absolutely no recourse in, no evidence supplied, none required... where is the democracy / discretion / sense in that!?