Senua, exactly. It really suits my dyslexic DD to be able to drop all essay subjects at 16, whilst a different child might opt for IB (which is offered by Sevenoaks). I am glad we have the choice.
And to pick up Grauch's point. Of course there are bursary students at a school like Westminster. All schools, state and private have different tribes within them. The cool gang, the nerds, the sporty kids etc. They will differ by school, but the principle is the same. The joy of Westminster is that it is fine to be a nerd (just as well in a school where 50% of the year group opt to take double maths at A level) but they have "the rowers" and "the safe group". (For those without teenagers of the right age, "safe" means cool.) Neither DC rowed, played rugby or was safe. They both have lovely friendship groups where money, possessions or backgrounds did not seem to matter much. DS has never understood why you would own more than two pairs of jeans and 4 T shirts, and sees shopping as a once a year chore. His friends are similar. The fact that one might have a billionaire father, and another was living in an ex-council flat with his single mum was really neither here nor there. (And as much credit to the billionaire father as the cash strapped mum.)
Teenagers are greatly influenced by their peers. Perhaps H. Holle might consider instead who his children choose to hang out with, rather than blame a school for a lack of diversity. As Kindle suggests above, you can have children of Turkish migrant families within a school but it does not mean better off German teenagers mix with them. In all schools I am sure there are kids who give weight to possessions, and in private schools the bar will be high, far beyond having the right brand of trainers or the right school bag. Ditto most schools will have a party crowd, which if you are at a private school means a child's Facebook feed can resemble Rick Kids of Instagramme. (There are only two or three degrees of separation within the London private school world so DD will know people who party with the rich and famous - even if she does not.) H. Holle might not be seeing diversity but it will be there. Sevenoaks will have plenty of straight forward day pupils whose parent work hard to pay fees, there will be bursary children, there will be teachers children. We know four families whose children were or are at Sevenoaks and none are from H. Holle's rich London ex pat banker world.
Different cultures have very different expectations about education. European cultures are broadly aligned on the relative emphasis between academics and other aspects of teenagerhood, say compared with some Asian cultures. But there are still differences. One mother managed to bemuse me by starting off complaining that Westminster had not offered a place to her second child, but then continued to complain about Westminster itself. Her verdict..."it was too English". This may be H. Holles other problem. He thinks he will be buying a superior German type education, but instead is buying a well regarded British education. Which is better probably depends on what your expectations and priorities are.