Frogs I would never speak about schools in quite the way Xenia does - children being "thick" or "not clever enough" etc...
However I have ds1, y10, at W and ds2 to follow next year. Ds1's best friend from his old school is at UCS. I know a few parents with boys at the Junior Branch too. Ds2 sat for UCS too and was offered places at both.
My observation over the past 14-15 months is that ds1's friend at UCS spends just the same amount of time, if not more, as ds1 does on his schoolwork. For example he usually has 4+ hours of work set over the weekend, whereas my ds1's weekends (after Saturday morning school which admittedly took some getting used to) are COMPLETELY free for him to unwind. My ds1 is bright but not a genius; W suits him well and he isn't stressed. At the time of his interview a member of staff commented that the boys who are happiest there are the ones who shine at one or two things, whether it be maths, art or drama. This has proved to be the case with ds1 who's beginning to find his niche in art/creative writing/history, without being in any way a driven or overworked boy.
At UCS the vibe I get from parents is that the place is very laid back...which (without being inflammatory, as I'm finding W more laid back than I'd been expecting) has its pluses, but it does mean that perhaps the very brightest boys aren't being stretched enough ? Certainly for a selective school in a very middle class area with most parents being well qualified professional people, the percentage of Gcse A grades isn't as high as one would expect (38% in the Times Parent Power league). If A is the new A, and if dozens of schools, state and private, can better this, while still offering lots of extra curricular trimmings then personally I'd be looking for more in terms of added academic value.
But that's my personal opinion.