It never was the law but a case of avoiding bad PR by being pedantic ( would you help a plod after being done for 31 in a 30? or even 30.1 in a 30) and policing by consent. The police want to catch / stop the idiots not those trundling along at or about the speed limit. Which is also why fixed camera sites are bight yellow and temporary locations are announced beforehand. And in a Freedom of information investigation, 26 out of 39 constabularies said 10% +2mph and the others didnt answer the question.

Reality of course is that if you fail what a patrolling officer might call "the attitude test" you could well be done at any speed for almost anything.