I teach, and have taught, in south west London and Surrey. In both independent and maintained.
When you say ‘top’ schools, I assume you mean the likes of Westminster/St Paul’s where it is worth investing in a feeder pre prep for these schools, as they know the drill regarding entrance exams and will advise if tutors are needed. There’s no point being pushy, but you do have to be very invested in your child’s education to get into (and thrive) in these schools.
Other than those very top schools and feeder schools, which are completely shielded by reputation from the recent VAT hike, I would say that the majority of pre-preps do exactly the same thing and are struggling a bit at the moment. One school I know, a very nice school, has a reception class of four pupils!
Unless you have money to burn, or are particularly keen on four year olds in blazers, I would stick with the old saying “state before eight.” From year 3, I think the advantages of class size/facilities/after school care in the independents really do make a difference to the outcomes for an ‘average’ child.
Just be aware that your child may need to do some extra work outside of school for the 7+/8+ entrance tests (if they have to take a test at all) as there are aspects of these tests that are not part of the state school curriculum. But dead easy to look up online. And there are plenty of schools keen to fill places at the moment.
Don’t really know about the parents. They all drive Range Rovers, but are mostly nice, in my experience!