I can only talk about experiences of my own. Not facts. I no longer teach secondary. But I do know of real examples of my own, but that's not stats for you.
And in my experience (form my own childhood, 10'years teaching exam classes in the past and currently my own dd and her friends) kids generally do not take mocks anywhere near as seriously as the real things and many, if not most, do increase their grades - some greatly so - between mocks and the real thing, as all phases of education. Kids revise for mocks, yes. But no where near the same extent. Dd did revise for mocks but when the real things came for GCSEs the revision massively increased. She was no longer learning new material and she was focused on studying and recapping and revising, all day every day. Not having Christmas and Boxing Day fun, fitting in revision amongst homework and coursework and being in school for lessons. It was a huge change. Her friends were all the same. And almost all their grades from GCSE mocks went up by a minimum of one grade per subject. The mock grades didn't include their coursework elements either, just the exam grade. Clearly some children are way better at coursework than exams.
Not statistic proof but my own personal experience (not friend of a friend)
My a level economics. January mocks - E. Real thing in the Amat - A
DD's mock maths GCSE in the January - a borderline 4. Real thing - 7.
I taught many students at gcse and a level and almost all had different real grades compared to mocks, and many did go up.
Both myself and dd were/are good students and revised. But the effort after mocks and once the courses were finished and focus could change was huge.
Our experiences are, in my own life experience, not unusual.
Mocks would NOT be a fair representation of pupil arraignment.
But the government, OFQUAL and schools know this. Hence they aren't asking teachers to rely on them anyway.
And my point still stands; many, if not most, students do not take mocks any where near as seriously as the real thing. Many learn from the shock of a bit so good mock grade. Schools, likewise, often don't take mocks as seriously as the real things.