Mock driving tests are popular with some instructors and very popular with others. I have my reservations and think that sometimes pupils and instructors get hung up on performance in mock tests.
Thinking back to my schooldays I remember the period just after Christmas when mock tests were undertaken in every subject. They had two aims: one, to give us (and our parents) some idea of the likely attainment and two, to give us some idea of what the actual exam conditions might be like.
I think driving mock tests should be the same. Giving the pupil some idea of the format of the practical test, the procedure, the form of words used and the likely pressure is a good way of alleviating some of the test day nerves. As far as the likely attainment goes, I think it gives a rough idea - but no more than that - but this is, or should be, for the benefit of the pupil!
However, this approach is not ideal because an instructor will always mark differently to an examiner. We are biased, they are not. We have previous experience of the pupil's driving, they do not. We are likely to be well-disposed towards the pupil, they are not. Any or all of these can affect the way that the mock test is marked, either to the advantage or detriment of the pupil.
I know some instructors subject their pupils to multiple mock tests until they can 'pass'. I think this is a flawed approach partly because of the reasons given above but also because mock tests should never be used to force improvement in performance. They can be used as a tool to give confidence to the pupil if necessary but the instructor should not need to use them to assess whether the pupil is ready or not - they should be ablle to do that without carrying out a mock test. Certainly they should never be used to justify more lessons for the pupil, something which I am aware does happen!
So I will conduct one, or maybe two, mock tests if I feel they are of benefit to the pupil, trying to mimic the conditions of the actual driving test as best I can. This is quite sufficient as far as I am concerned...