Mock test providers and tutors can only guess at what will be provided in the real CEM test. There are no past papers or details of what will be in the test so they are really shooting blind with a few hints from previous years.
The 8th is an actual CEM test done in the correct timings so you will put your dc at a disadvantage if you do not sit it and many hundreds do.
There will be educational tourists (who have no interest in applying for Bucks schools) who will travel many miles to sit because it is a free real CEM test which is worth a lot more than any mock or tutor can provide you with.
Using mocks and then not bothering with the real CEM prep test would be like paying to watch a tribute band when the real band is playing for free a few miles away. DG
My DC sat a Bucks mock prior to sitting the actual test. It was too easy and therefore not adequate preparation for the Transfer test itself. In the actual test one DC then became very distressed at the increased level of difficulty and sobbed through a large part causing distraction to the other sitters. Maybe this year will be different but thus far they appear to have used the same mock.
... and Bob is right it did give my DC a false sense of security.