Hi, OP -
I believe this is a preferred institution of the Spanish and Dutch royals, with at least 3 from these houses attending currently or in recent years.
UWC appears to offer a high quality education aiming to develop the whole person. Kurt Hahn who founded Gordonstun (sp?), which wouldn’t sound right for this girl, was closely involved with it and the emphasis on sport, volunteering, teamwork etc reflects his theories. I don’t mean to say that UWC is stuck in the early 20th c, only that it doesn’t offer an exclusively or perhaps even primarily intellectual focus.
Nevertheless UWC offers an IB programme (you would need to check for A levels) which is usually a sound marker of intellectual quality. My guess is that it wouldn’t be a disadvantage for someone aiming for a top STEM school, but I don’t think that intellectually it is necessarily a huge advantage.
However, what can happen when you find a good fit and everything suddenly clicks is that the world opens up. You can learn to learn, start doing significant independent work, etc. Oxbridge and the other top STEM schools (traditionally Imperial and Warwick, but this is changing) rate this kind of thing highly. It sounds like this may have happened to the person you know.
I had a look through some old threads and some of the boarding schools that came up in discussions of academic rigour were Rugby, Oundle and Marlborough. Also, perhaps Westminster has a coed 6th form? I don’t know whether these schools do IB, however.
I am by no means against UWC. It sounds intriguing. I think there is much to be said for offering young people a rounded life and letting them find their own priorities. However if this family is looking for an environment where many are aspiring to Oxbridge and other top programmes, it isn’t clear to me that they will find it at UWC.
I, too, would like to hear from someone who knows UWC better.
(I have done a certain amount of reading between the lines - I have some experience with this, as a former STEM admissions tutor in the tier just below COWI. I have seen applications from the schools mentioned above, and others, but none that I can recall from UWC. This is in no way significant)