GHS has always done well by creaming off the top of its applicant pool and then production line them through to public exams. A best friends daughter is there currently, and, compared to her sister, who didnt get in, and ended up at another school, she isn't having as 'good a time'. It get's great results (at the moment), but given the initial creaming, there is no need for any significant 'value add', so my friend says they don't really focus on all those 'scholarly type things' as much. That said, it does what it says on the tin and gets good results.
On Oxbridge, there are other factors at play, If you look at the stats, there has been a big focus on diversity at entry, so the london schools with more ethincally and financially diverse cohorts have tended to get more in as a factor of that. This, as you may have seen in the newsn is changing a bit now though. This won't be the only reason, I dont know what their university application provision is like at the moment, it could be that effecting it too.
Also - Coffee shop mums chat is that GHS is now filling the school up, lots of actual reports and second hand info that girls being accepted are those that didnt get offers elsewhere locally and were genuinely shocked to get in.
Its also rumoured that they've opened up another year 7 class. It seems like they're maximising their income streams, probably to support the rest of the groups finances as we know United Learning have struggled recently - one example is the Royal School (had a once removed mum friend there too) which had to close down, and due to its location and covenants, is almost useless as an asset too.
Might be worth asking about their future plans on numbers, as if you pack-em-in on such a small site, things will undoubtedly change.
PS. Do note that those who do go there, it is the mecca of education and must not be criticised, at all, so be wary of that fact! And that comes from my mate who has a DD there!