A pupil's driving is well below driving-test standard. What should you do if they insist on taking the driving test?

If your pupil's driving is below the required test standard, it's your responsibility as a professional instructor to advise them not to take the test. As soon as you become aware of a possible problem, you should discuss this with your pupil and positively encourage them that, in their interest, and for other road users as well, it would be far better to postpone the appointment. If you can't dissuade them, then you should withhold the use of your vehicle. Ultimately, it's the pupil's test and an instructor can't prevent them from taking it in someone else's vehicle.