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.
Unless otherwise agreed with your instructor, the test day hire will be scheduled into the app as a 3-hour session, including 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.