Sorry - my answer was probably a bit choleric - Christmas cabin fever. WUS has always sent about 8-20 of their 42 odd original boys (oddly, the last few years one or two of the boys joining at 11+ have gone, which I think is rather naughty) and whilst I am sure they would prefer to keep most of them, the school has always acknowledged that it is hard to know at 7 where a big will ultimately go, and WUS is after all a prep school.
It may change eventually I suppose, once the no CE for Great School kicks in, but personally I doubt it. Equally, eton is over turn going to have to think about how it deals with CE I’d more schools join westminster, wellington etc. Currently I think they are highly opposed to dropping the requirement.
WUS boys will be well equipped in the core subjects and languages to take CE, however WUS adjust their curriculum. For the more rote-learning subjects, notably history and RS, if wus moves away from the rigid CE template over time I imagine the destination schools will adapt.
I guess st Paul’s boys going to eton have been in this position for a while, but some still go.
How it would work from other genuine through-schools (especially ones straying at 11+) I don’t know.