In order to book the car theory test, you must be 17 and older and have a provisional driving licence. You can book your theory test date online at the official DVSA website. Click the link here to book - https://www.gov.uk/book-theory-test.
If you arrive late, you will not be able to sit the test. Plus you will also lose your test fee. So, make sure to note which test centre you’re to go to and be on time with the required documents.