There are frequently children who go to Harrow, Eton, Westminster, Downe House, Cheltenham Ladies etc from these schools and even win scholarships.
Please where is your data to support this?
Both schools are very good at meeting the needs and expectations of parents when their children move on at 11+ and 13+.
I don't see how they can be good st meeting expectations of those leaving at 11+ when their focus is to prepare for 13+.
Of course most girls will leave at 11 at most 13+ prep schools, that has always been the norm, but where the data showing all the top indies they move on to?
I take your point leavers destinations will be partly driven by parents own choices, but I would expect children from local preps to at least be feeders into the local senior schools e.g St Albans, MT, Habs. Both schools are known for sending the few odd, they are not feeders into these schools and its not for lack of trying. Apart from the fact their leavers destinations page is telling (a list of various schools pupils could go on to, as opposed to more meaningful year by year by school figures) I bump into parents from both schools regularly with dc in the upper years.
They are both beautiful schools, and I'm in no doubt a child as young as OP's will thoroughly enjoy it. But it's usually once upper school starts that it starts to become clear that their academics may not be sufficient in getting children into certain schools. However if OP is not interested in any of these schools there are others (as previously suggested above), although I would wonder since a school like Berkamsted has its own prep why not just go there in the first place and avoid all the drama with 13+ schools. What is the point of going to a 13+ prep school, if top independent schools that start at 13+ are not on your radar?