In most speeding cases you will have received a Notice of Intended prosecution (NIP) through the post, and you will have filled in the Section 172 part of the form nominating yourself as driver.
In cases where you already some or no penalty points and your speed over the limit was low, then you may be offered a Fixed Penalty fine by the police or even a speed awareness course which involves no penalty points. In this scenario, you effectively plead guilty to the charge by accepting either the fixed penalty or the speed awareness course. However, do remember that in either scenario the prosecution is under no obligation to notify you of the evidence that they apparently hold against you.
However, you have every right to plead not guilty to the offence in which case the prosecution are obliged to let you have sight of all the evidence that they intend to bring to court in order to make their case.
In these situations it is recommended that you seek the advice of a specialist motoring lawyer. It may well be that the police and CPS have made no mistakes and you are happy with their evidence in which case it is perfectly acceptable to change your plea before going to court, but there may be procedures and processes that have not been followed correctly in which case a motoring lawyer, like Julian Harris, will be able to then exploit such mistakes to your advantage. Either by trying to halt the prosecution before it goes to court or by seeking to persuade the Crown Prosecution Service (CPS) that a mistake has been made and that the prosecution should be dropped. If this does not succeed then the same arguments can be used at court in an attempt to plant sufficient doubt in the minds of the Magistrates, leaving them with no option but to acquit.