The DSA website explains what is involved in the practical driving test. This is what they don't tell you!
Driving tests are taken in the vehicle that you learned in. This is generally your instructor's car. If your instructor does not think that you are ready to take the test and you have booked it anyway, he or she may refuse to let you use their vehicle. Instructors have to maintain their reputations and it is bad for their business to present someone for test who is not ready.
The examiner is employed by the DSA to assess your driving. They do not have quotas, they do not get paid according to the number of people they pass or fail. They are paid a salary and will pass you if you fulfill the criteria and they consider you safe to drive. Examiners also have to maintain their own reputations and that of the DSA as a whole so if you commit a serious or dangerous fault they have to fail you, even if the rest of the drive was perfect.
The driving test is marked on a sheet. The examiner may make marks on the sheet as you are driving. Don't let that distract you! He or she may just be making a note of something to mention and even if he or she is marking a fault, it may not be a serious one.
You are allowed up to 15 driving faults but any serious or dangerous faults will result in failing the test. If you get three driving faults within any particular category that will become a serious fault.
What is classed as a serious fault or simply a driving fault is often dependent on the circumstances. Failing to check a mirror before signalling to turn right may just be a driving fault if nobody is affected, but if a motorcyclist had to swerve or stop as a result of you not knowing that she was there, that would be classed as serious. Equally, in some cases an examiner may wait until the end of the test to see if she wishes to class a fault as serious or not, depending on the rest of the drive. A dangerous fault is one where the examiner felt he had to intervene to avoid a collision. In extreme circumstances the examiner will abandon the test, basically if he no longer feels safe in the vehicle.
Examiners are human beings too! On the whole they want to pass people and want to put pupils at their ease. You get the occasional bad one and there are complaints procedures you can follow. Even though the best you are going to get is a retest without being charged again, it is worth complaining if you genuinely feel that the examiner was unreasonable in case there are problems with that particular examiner and the DSA need to know.
Finally, whether you pass or fail, it is useful for your instructor to hear the debrief at the end. And your instructor will always drive away from the test. This is nothing to do with insurance, it is simply that driving tests are stressful and whether you have passed or failed, the chances are your emotions will be heightened and that is never a good time to drive!