Committed parent, I am a bit surprised at the 13 applying for Oxford. I assume at least some of the applicants were chancing it a bit, eg hoping they had a good day at both MAT and interview. As far as I know, Westminster will warn students that they are not likely to get an Oxbridge place, but does not stop them from applying. Some then gain places. It is after all, only one line on the application form.
(FWIW good computer science degrees seem particularly hard to get onto.)
Westminster offers the advantage of a sort in that students tend to know how good they are. Following normal distribution, each year will can expect small number of really outstanding mathematicians (0-3?). Others, who might have been star of the class at other schools, will be aware of their limitations.
Our observation was that many then appeared to opt for maths-heavy degrees (engineering, physics, nat sci, economics) often putting Cambridge first followed by London. In DS' year Cambridge was tough. From the outside, maths ability seemed to play a large part. Top set mathematicians got their economics or engineering places, but some very able students bringing a lot to the table, did not. Most of DS' friends were more than happy with the courses they ended up on (Imperial seems to offer more specialisation), one or two decided to have another crack at Oxbridge, another simply had not decided what he wanted to study, and inevitably the year group had some who either did not put the work in or cracked under (parental!?) pressure.
I assume Oxbridge places are becoming very competitive. Westminster is selecting some students aged for 7+. The amount of tutoring for 7+, 8+, 11+ and 13+ pre-test has risen exponentially. If pupils at Westminster are not necessarily brighter than before, and Oxbridge places harder to come by, overall numbers of offers can be expected to drop. That does not mean however that the school is not providing a first class education, nor that pupils will not go on to suitable, challenging, and well regarded courses, either in the UK or overseas.