You are also missing that it is the girls who decide and make successful applications to US universities, not the schools. That depends on them having the motivation and qualities (and money) to get in and for US universities that is an even more idiosyncratic process than Oxbridge. You have to make individual applications which require you not only to do the SATs tests, possibly including for specific subjects, but also to produce essays etc specifically to meet the specifications of each university. There are also all manner of strategies, early acceptances, sports scholarships (that eg recruit the top LEH rowers, but not always top academically
, to Ivy League) etc to get in. Even the US expats I know who have gone through the process have to do a considerable amount of research and individual preparation. I am sure all these schools will respond to the increasing trend for girls, especially in affluent West London, to want to embark on that intimidating process, with sessions by Consultants , contact with old girls etc. but they cannot provide the individual support preparing for SATS etc.
I absolutely would not be making prep for Ivy League unis a consideration in choosing a west London school, a complete blind alley. Lots of factors already mentioned here are worth considering, and above all, where your DD feels she would be happy.
Also I disagree with Needmoresleep slightly, which I don't normally. Knowing the US expats who have been successful from other parts of the world, the fevered preparation in extra curricular that happens in the UK does not seem to happen there, perhaps because there wouldn't be the opportunities. Rather they target certain unis and come up with strategies to maximise the strength of their applications for those. One of my friends did remark " it isn't good enough to be rounded these days, you have to be rounded with square edges and sharp elbows"
I think as with Oxbridge that requirement for huge amounts of extra curricular valour may exist largely in the parent's minds, unless they know their DCs are not going to get in on intellect alone and their strategy is to go for a sports scholarship, but then IME the talent scouts come to you, not the other way round.