Increased fines have come into force today in England and Wales for speeding offences. As this has been in the news a lot, many people now believe that you can be prosecuted for going 31mph in a 30mph limit.
You always could be prosecuted for 31mph in a 30mph! But it's not very likely.
Speeding is an absolute offence: you're either speeding or you're not. However, police forces usually allow for tolerances in equipment and are unlikely to prosecute for such minor discrepancies.