You have a duty to provide ALL information within your power as to who was driving.
If you don't then you commit the offence of failing to provide the driver details per s.172 RTA. This carries 6 penalty points and a hefty fine.
It is not for the Police to identify the driver it is for you if you are the owner/ registered keeper of the car. Generally people will know who is driving their vehicle e.g. if you loan it out to someone you would be expected to know their details.
If you have provided all of the information in your power to give then notify the Police of this.
If the Police think you are holding information back then they will take you to court. You can have a trial if you don't agree you have committed the offence of failing to give information and if the Magistrates agree that you have given all the information within your power they will find you not guilty.
If they think you failed to give all the information then they will find you guilty and you will receive 6pp, a fine and an order to pay costs etc.
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