If you've been having lessons with an ADI, you'll use their car just like you would for a lesson, I'd hope if you've been having lessons with an ADI you've checked that the date and time you've picked is free for your ADI, if it's not and he won't cancel the lesson he has when you have your test you can't use his car.
If you need to use your own car for whatever reason you need to make sure you have L-plates and your insurance covers it, for the purpose of the driving test, you are not driving under supervision, the examiner is there to examine your driving and nothing else. If you take your own car you MUST also take along someone who has had a drivers license for 3 years and is over 21 to sit in the back and "supervise" otherwise your insurance is void and you can't take the test. You MUST have your L-plates on, you MUST have supervision and you MUST have another interior mirror for the examiner to use.
http://www.direct.gov.uk/en/Motoring/Lea… Cars that can no longer be used in driving tests for safety reasons
http://www.direct.gov.uk/en/Motoring/Lea… Minimum requirements for all cars to be used for practical tests