Its not widely advertised because its not true. Speeding is what the law calls an offence of strict liability, in other words 30.0001mph in a 30 limit and you could be prosecuted. However a tolerance is used when setting speed cameras and deciding whether or not to prosecute.Its not widely advertised (for obvious reasons), but there are tolerance over speed limit, only above which you can actually be *prosecuted* for speeding.
There are two main reasons why a tolerance is allowed. Partly because its not realistic to expect drivers to manage that degree of accuracy and enforcing it would mean people driving well below the limit with consequent adverse effect on traffic flow. Partly because any speed measuring measuring device will have a small error and by giving a tolerance bigger than the machine error removes a possible defence.