@MissMeYet , be over prepared for the entrance exams. Three of DD1's friends were offered places over the years, and the one thing that stood out was their work ethic, and their ability to understand concepts quickly. Anyone applying will expect to get high grades at GCSE, that's a given. But, more importantly is reading beyond the curriculum.
You have nothing to lose by asking the admissions team and the head of sixth form the same question. They should be able to guide you in the right direction.
My DD went to Westminster from PHS a few years ago. Quite a few girls applied from there, all of whom achieved all A*s at gcse, but only 2 got interviews and offers. I have absolutely no idea why the others weren’t selected. I think they all received offers from KCS.
I know this isn’t very helpful. The one thing she did say was she felt the interview was a formality as she got the impression they had decided candidates in advance.