As an 11+ mum a few years ago, looking back, these are my sincere advices. Firstly, pick a school where you expect your child to be above average, preferably in the top 1/3 or 1/4. It is not good both for the child and for the parents if the child is way below average. You would not have the same resources or the same motivation if you are in the bottom set. The child will most likely get better results and better uni if he goes to a school where he is above average, preferably among the top 1/3 or 1/4. Think carefully whenever you apply to an aspirational school.
Secondly, always go to a school where the travelling time is reasonable, not more than average of the students attending that school. It doesn't have to be the school which is closest to home, but you child should not be one of the few who lives very far away. He or she would be very tired, coupled with being among the bottom sets, severely affect his or her academics.
I made the wrong choice a few years ago, choosing a very top school (always in top several) and living far away. If time could turn back, I would choose the 2nd choice (a school usually ranked 20-40 but still very good schoolwith 10% a year to Oxbridge and most others to good uni), where my child would do better, be happier, and are us parents.