Get labelled incorrectly in grade 8 and get shutout of any advanced math/science classes which disqualifies you from a good university program.
I don't know how it is in the UK, but your comment strikes me as a typical boogieman from someone who hasn't actually seen the system in action.
The school system has as a track of classes leading up to calculus. A student can jump forward on the track by passing a test on the next class in the track.
Because there were other demands on my son's time, it was not practical for him to learn a year's worth of material outside of school time.