SORN stands for Statutory Off Road Notification and it's used to tell the DVLA that your car isn't parked or being used on public roads. If you think you won't be driving for a long time and don't want to insure the vehicle at all, you will need to make a SORN.
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Do remember with a By Miles policy, if you don't drive, you won't pay a penny that month. Your car is already covered by your fixed annual cost while it's parked.