Summerends, a cool table.
Tiffin provides a interesting comparator in that it is probably (huge catchment for the years in question, free, much higher applicant numbers, supportive and aspiring parents, and taking in additional achieving students for sixth form) more selective than Westminster. Yet "only" 130 applicants over the three years and a 33% sucess rate. Comparable figures for the girls school are 84 and 33%. (W's figures are 319 and 45%.)
I also agree with OxfordComma about the impact of only being able to apply to one of the two. Observation was that those wanting to do mathsy subjects and with a credible application, were opting for Cambridge/London/other RG. Lots ended in up in London. Other, arguably no-stronger candidates, got Oxford offers.
I also agree with earlier comments about Oxbridge being full on and having less appeal for some students. A friend's DD, at SPGS, decided she wanted to avoid Oxbridge. Having worked hard for the entrance exam, working very hard whilst at school, and envisaging a high powered professional career, she wanted a few years out of the private school/Oxbridge/City pipeline, and opted to go somewhere which ranks highly for student satisfaction. My friend thought this was a great idea, but noted that the same would not have been an option for several of her daughter's classmates.