Going for the Driving Test can for some students be a very nerve wrecking and challenging time. Hence it is not a good idea to rush it otherwise it can result in unexpected pressure and silly errors. Driving Test in my opinion should be booked in agreement with the Instructor, and changed according to preparations. I mean what is the use of going to a test under prepared and spending more than £100 "to see what it's like" when all you need is a mock test with your own instructor or their colleague. I have seen many students going to see what it's like and eventually the penny drops and they realise that maybe they are not prepared for the road. This can be either mentally or physically. I mean mentally not strong enough to drive under pressure whilst being assessed or physically unable to control the vehicle safely in that situation.
Preparation is the key to passing the Driving Test first time.