There is the 10 percent plus two rule of thumb
That is to say that you can be travelling at ten percent plus two miles per hour over the limit and will not ordinarily be stopped/photographed so for a 30 zone it would be 35, a 50 zone would be 57 and so on. I believe it has something to do with the legal requirements for the accuracy of speedometers
Having said that, travelling at even 1mph over the limit is still over the limit and if you get caught then you only have yourself to blame.