My view is that it is happening a lot. My Dd was at a fairly small, non-selective prep. It may be the same as some other people on here. She definitely told me that after the exams they were asked to put their hands up in her maths group if they had had tutoring and there were only two of them who hadn't, her and one other (whose father was a maths teacher!) They were supposed to have their eyes closed when they did this but DD looked (that's my girl!) She relayed this to me with some indignation, as though she had been short changed by not having a tutor!
Its a bit chicken and egg-y. I didn't think DD needed additional tutoring; she seemed to be getting results that were pretty much consistent with every report she had had and didn't have any particularly obvious weaknesses or blind spots. My view was that tutoring in those circumstances would have been bonkers. As it was, she got in to all of the schools applied for, and it was a pretty top-end selection, so we were right that tutoring really wouldn't have added much into the mix. I do know that its pretty easy for me to take the moral high ground when you have a dc who is performing well - not sure what I would have done if I felt panicked that she was borderline on a few things.
But there were some children who I know were tutored very intensively, including one who was specifically tutored to get into a school which the prep school did not recommend (the parents were quite open about it). It worked.
I felt the prep school were annoyingly ambivalent about it, although I'm not really sure what they can do. I know they feel it puts additional stress on the child, and that there is a risk of muddling them when the tutor uses different methods or stresses other things. We were given mild warnings that it was not a good idea in their view. But for the preps the leavers destinations table is their selling point, so I think they really don't care that much how the results are achieved and are happy to benefit from it, while not being seen to support it. Having said that I did hear from a friend who still has dc at DD's old school that the head recently sent round a very snarky letter telling parents to cease and desist, so maybe they are taking it seriously. But for sure it is very prevalent.