I agree that if you actually have the Tiffin offer and the independent schools, £20k is a lot to pay for the shinier facilities and experience which is a bit different, unless you are absolutely loaded and won’t notice the £20k is gone at all.
In reality, in terms of ability Tiffin will always be harder to get into as it’s a numbers game. So many apply per place.
Every year a handful get Tiffin who then go independent. They tend to be the people with vast vast incomes or those who are attracted to the independents by big bursaries and scholarships.
There are more clubs, and there will be nicer facilities and smaller classes and a parent body who are more established in affluence and so more willing to support the idea of degree courses such as History of Art or courses which don’t directly lead to a career, whilst the Grammars will have lots of parents who are very focused in medicine or law or very obvious career paths. The independents often dont have such an able cohort, but somehow squeeze as good or better results from their students, and interestingly, better uni destinations overall. But whether those things are worth £20k when Tiffin will deliver a fantastic academic education and a vast array of extra curriculars is a hard one to turn down, unless the journey would be crazy or money no object.
Coombe is highly regarded too. Clever girls will do really really well.
A lot depends on the sacrifices school fees will mean for you and whether you value the extras that independents will offer to the tune if £20k.
Personally, I’d think it was worth it if the state options were poor, but if the alternatives are Tiffin or Coombe, really not worth it. You’re paying an awful lot for a bit more, but actually not that much more.