It is totally variable and depends both on the schools and the parents’ situation.
The main features that distinguish private schools are smaller classes and a pretty homogeneous class environment (yes, there are a few bursary students, but very very few on full bursaries). In addition, there are a myriad of extra curricular clubs and support classes for struggling pupils.
The downsides, however, are cost and, in my opinion, a failure to develop genuine resilience for many pupils.
It is definitely true that, if you are someone who may be on target for a bunch of 5s and 6s in state, private will probably get you 7s and 8s, as you will end up in a small bottom set of 8-12 pupils, with fantastic support.
OTOH, if you are a resilient pupil on target for 7s to 9s anyway, you will be in a top set of 24 (sometimes even 25 or 26), not that much smaller than the 30 at state, and most state schools don’t have behaviour issues in top sets.
In addition, the savings on state schools, if parents have the time and interest, can but pupils far better extra curricular experiences at specialist external sports clubs, music tuition etc etc.
And private schools do spoon feed! There is a trade off between teaching inspiring science, say, and optimising GCSE grades. Private schools add extraordinary value at GCSE (partly by sitting the easier IGCSEs) but that is harder to carry over to A levels.
Ultimately it is about the right school for the right child and family.