I am fully aware of the guidelines the police use for prosecutions, speed limit + 10% etc.
But are you suggeting that you cannot under any circumstances be prosecuted for exceeding 30mph, say doing 33mph, in a 30mph zone ?
My Wife was pulled up by the police for, according to them, doing an estimated 33 mph on her motorbike. At that time, there were strange things going on in that area between the police and motorcycles. Eventually a policeman was jailed for beating up a motorcyclist because he didn't like his cocky attitude. things went back to normal after that. My wife was following a police car at 30 mph. It started to slow down and pulled close to the edge of the road, so she took it as a singnal to pass. As soon as she passed, the blue lights came on. It was night time so they couldn't see that it was a pretty girl. They were quite agressive until she took off her helmet. They said that at the time she overtook, they were doing 28 mph and they estimated that she was going 5 mph faster than them and that they could book her, but decided to let her off with a warning. She was petrified through the whole experience and still shaking when she got home ten minutes later.
A few weeks before, I got a speeding ticket in strange circumstances. I stopped at traffic lights in the inside lane of a two lane town centre road. A police car pulled up in the other lane. When the lights went green, I waited for the police to go first, but he didn't go. After a long wait, I had to procede as there was a queue behind me. On my motorbike in a busy town, it was easy for me to make progress through the traffic, but the police car was getting blocked, so my lead increased until he was about 1/2 mile behind me as I left town. I was very suspicious because of his behaviour , so kept looking in my mirrors to see where he was. Eventually, with him more than a half a mile behind and completely out of sight on a clear road, I turned left through a very small village out onto the open road and away. After about 20 minutes and around 12 miles, I could see the police car behind me again overtaking dangerously without blue lights on. He followed for about 10 minutes, then pulled me up and said that I had been speeding, which I hadn't. After some questioning, he changed his story to me speeding through the village. He said he was right behind me measuring my speed, but he wasn't. He showed me his vascar readout of 40 mph, which he could have put on at any time, like when he was speeding through the village trying to catch me up. I went to a lawyer, but he said that I would have no chance in court, so I was forced to plead guilty and put it down to experience. Not all policemen are honest, though most are OK.