First up you should speak with your instructor to ensure that you choose a date when their vehicle is available for you to use. A lot of learners find themselves in a pickle because they've booked a test date that their instructor can't accommodate!
As a general rule, we won't be able to accommodate late-notice requests for a driving instructor/vehicle for your test. Our T&Cs state that you'll need to give us at least 5 working days' notice for us to be able to arrange an instructor to cover your driving test.
Instructors will usually charge between 2-4 credits for test day and unless otherwise agreed with your instructor, the test day hire will include pick up before the test, use of the vehicle for the test itself, a debrief and a lift back from the test centre.
Unfortunately we can't get involved in the booking of your practical test - you'll have to contact the DVSA about that yourself. Head to their website and book a test slot that both you and your instructor are available for.