I would agree with other comments on King's Hall/King's College, and I find the scholarships policy disgraceful. KH is not the only prep in the region which has stopped preparing pupils for common entrance/ISEB pre-tests, which I think is dreadful as it actively disadvantages their pupils when applying to other senior schools in order to ensure they remain with their linked senior school. As I say, KH is not alone in this, so do check whichever prep you choose, unless you are 100% happy to commit to the senior school the prep feeds into.
In rugby this year King's Hall was definitely the school to beat in the region, at least for my DS's school year (in the last match of the season we celebrated the fact that we lost to KH by only one try!). That said, last year the same year group was much weaker in hockey and athletics, so I guess it does vary from sport to sport. I don't remember their performance in cricket, though I know this is a major sport for King's College itself.
When we were looking at preps it was difficult to distinguish between the academic provision of various schools, so my earlier comments were based on other aspects of our visit, which may matter less to you.
We were in particular looking for a boarding prep, and we found the boarding provision quite weak. The boarding house was tatty with peeling lino flooring and a generally down at heel feeling (in our view) and arrangements for the boys seemed quite chaotic compared with other schools we looked at.
While the grounds themselves were impressive, the walk from the school to the sports pitches involved walking past a whole load of broken down, rusty gardening/grounds keeping equipment that was clearly no longer functional and in my own opinion would be frankly dangerous if (god forbid) a 10 year old boy decided to go exploring. Not a massive issue I admit, but not a good look for a 25k+ a year boarding prep. I have attended numerous sports fixtures at KH since then and the pile of decaying machinery seems to be growing rather than shrinking, so it is not a case that we visited on a bad day.
The lessons we visited did not seem to be well controlled, and there was a general feeling of chaos.
However, probably the biggest issue was that we found the head entirely uninspiring, and if you view the head as setting the standard for the school then that probably explained everything else we saw. Compared with the other preps we visited he was the least interested and the least accessible. When we did see him he seemed more concerned with talking himself up than hearing about our plans. Once he heard the prep school we were moving from (which has a very impressive head) he was very keen to emphasise that he and the head of that prep were good friends and (to use his own words) were "cut from the same cloth", but when we discussed options with the head of the school we were leaving we did not get the impression that he either shared or appreciated that comparison.
So nothing catastrophic, but lots of little things that added up, especially when we were comparing KH with schools that didn't have any of these downsides. A private education is not cheap. You have the right to be choosy.