I live in an 11 plus area, bordering another 11 plus area with a number of local high performing private schools. However, I have lived in this area for quite some years so I know parents with DCs in most of these schools. Believe me, the stats. on a piece of paper do not give the full picture and the OFSTED reports are nothing short of useless.
I've lost count of the number of times people have told me the school their DCs are at is not as good as it seems in terms of the quality of teaching and the level of learning support. Yes they all do quite well for the very top end of the academic ability scale, but those children who do well at these schools don't need the same level of learning support as those with lower academic abilities. For ability levels below the very top end, not so good. One very popular school in the area is on a four year decline in GCSE results with a less popular one on a four year upward trend in GCSE results. Interestingly the main change at these schools is the ability intake.
As it is an 11 plus area an extremely high number of children have private tutors from reception upwards in both the private and state schools. There is a lot of movement after year 2, whereby parents move their children into preps. to groom them for the 11 plus exam (a number of these parents continue to use tutors also). Those same children will then have the fall back option of a private school if they don't get their preferred academically selective state school.
One point to note here, in some areas where state schools appear better or there are academically selective schools, you will still need to be able to afford a private prep., a massive mortgage to fall into catchment, private tutors or the cost of the materials to tutor yourself. Yes some do get by without this cost but the numbers of such are very, very low.
Something that might be of interest to you is the DfE performance tables. This gives a clearer picture of how a school performs in relation to the various ability levels.
Just because a school gets good results it doesn't necessarily mean the teachers and management team are better than at other schools. It could be that the actual intake is more capable of performing with less support and guidance from the actual school due to high academic ability, use of private tutors and significant levels of extra support and interest in academic performance from parents.