My DS was at a small pre-prep which was able to differentiate and push him appropriately. However, because his interests were so different from all the other children, he had no friends - acquaintances, but definitely nobody he felt any similarities to. As a result, he had very few playdates (would prefer playing with the other children's toys or talking to the parent rather than playing with the child) and would spend many playtimes alone creating games.
It was principally for social reasons that I chose to have him apply to the superselective prep schools.
In terms of the questions asked, my DS has a year and a half left at the prep level, but it transitions seamlessly into the senior school so I can answer based on my experience and my friends with children further up:
High ability is the expectation, and any child without high ability would be miserable due to the pace and scope of the education given.
Very little busy work - DS has had very little homework, and still continues to have little compared to other prep schools.
No expectation for children to tutor their peers - I never heard of this before.
I have never experience NC levels and have no idea what they are. In terms of % of work outside NC, I assume loads as it is a private school which prides itself in not being tied to the NC.
Every child, even the jocks, have to be "nerds" as they are all academically able. There are, however, some children who are particularly focused on academic subjects, but they are not pointed to as being nerds.
Well, it is superselective, but even then it sets. Maths starting Year 5. French and Maths in Year 6. French, Maths, Latin, and English starting Year 7. By the time there are in senior school, there are at least 8 maths sets. To put this into context, the bottom maths sets has everyone getting an A* or A in the GCSEs except maybe 2 people.
Academic achievement is expected. Even the average student in the year group will get all A*s with one A on GCSEs.
All forms of achievement are respected and recognised (music, drama, sports, academic extracurriculars).