Regarding hothousing and my points above, if you look at GCSE results for, say Surrey, which has a very large number of privately educated children:
www.bbc.co.uk/news/special/education/school_tables/secondary/11/html/eng_maths_936.stm?compare=
The top 22 schools were all private.
The 23rd school is a Catholic state school.
The 24th and 25th are again private.
The 26th is a state boarding school.
The 27th is a private school.
The 28th is another Catholic state school.
The 29th is a state comprehensive.
The 30th is another independent school.
The 31st is a (mostly) comprehensive girls school.
The 32nd is a Catholic state school.
The 33rd is another independent school.
The 34th is another independent school (note, it's listed further down, King Edward's, but they do IGCSE: www.kesw.org/en/studying/exam-results)
There is another independent school listed further down, Box Hill. They are rather cagey about their 5 'good GCSE' grades. www.boxhillschool.com/about-us/exam-results/, but again this is an IGCSE school.
(Hurtwood House is a sixth form)
The county's state school average was 63% of children getting 5 'good GCSEs'. (Nationally it is 58%)
This average was surpassed by all of the county's private schools.
Not just one or two, but all of them.
And, don't think that these private schools are all for the super-bright. Clearly that is not possible. Just because you can afford to pay school fees, doesn't mean you have a very able child.
For example, the head of one of the private schools there with 95%+ 5 good GCSEs told me they didn't have any very bright boys at all, and that GCSEs were a spoon-feeding exercise. The Good Schools Guide, for what it is worth, confirms that in one of its telling asides.
Some of the schools in their lower down the list such as Box Hill, Ewell Castle, and so on, have a reputation for taking SEN etc.
Because private schools don't have a catchment, especially in an area like Surrey with so many to choose from, they tend to be highly stratified by ability - the brightest children will go to the most highly selective schools, and so on down the line. Obviously there are exceptions to this, perhaps you live close to a particular school, or want to educate two siblings of unequal ability at the same school, but by and large this is how it works.
For that reason the lower schools on the list will tend to have an intake skewed towards the less able children, and the fact that they still perform better than state comprehensives considered highly successful suggests that GCSEs can indeed be 'hot housed', if that is the correct phrase.