With a DS in Year 4, I would be interested to know which primaries have the best success rate in getting boys into KES and what is so good about them.
Notwithstanding any indies that might offer 11+ prep in school! surely the success of any state primary getting it's pupils into GS is far more to do with the parents and what they do for the child out of school, than in it. Given a bright child and the right sort of coaching (by which I mean minimal to understand the exam questions and to back up curriculum work done at school), success in achieving a GS place is much more likely, but has very littles do witht he school. In our primary, roughly half the small year group of 17 got GS places, but the year before it was much fewer. In a small school statistics for how many get through in each year group are meaningless as with such small numbers the percentages can vary widely.
I think you look for a school with good added value scores, then back up your sons primary education in year 5 with minimal coaching and practice to be familiar with the exam. I don't think you can pick a school more surely than that.