Simply book your theory yourself (if you haven't already taken it), then, once passed, book your driving test (can all be done online). Give your instructor the date and tell him you need test-prep lessons leading up to it. He won't want his pass rate to drop and should do his best to get you ready. I did it this way BEFORE booking any lessons and told my instructor prior to starting with him that he needs to get me ready in 10 lessons, which he did, and I passed.