I stated in one of my comments I don't actually want my kid to go to Oxbridge. My kid has to decide where he wants to go himself later.
I have friends who have children that got offered to Oxbridge but they declined at the end, and went travelling and then decided to join the universities in other countries instead (US and Germany).
I don't want my kid to feel too pressured being in the school, hence I don't want to get him heavily tutored for the exam. My son is the least competitive kid I know (among my friends' children and his classmates), he just doesn't care and does what he likes.
You either get the kids prepared by the schools, or hire the tutors, or do it yourself, but the kids are going to spend extra hours to study by the sound of it.
I'd rather let my kids do the hard work in school, than spend time in school thinking they can't get much from there and then have to get extra tutoring at home.
I was one of the bored kids in schools because I didn't get much stimulation from a normal school. They have to spend time helping the less able kids to catch up with school work, leaving the more able kids have nothing to do.