@PreplexJ yes, but being in your academic cohort is possible even for academically brilliant children at most selective schools in London, not just a few.
Many schools in London for instance have a reasonable number Oxbridge / Ivy league students each year. For some schools, this will be the equivalent of the top set and teaching for this cohort within the year group will be adapted accordingly.
For SPGs, this might be the equivalent of the top 2 sets (not the entire year group anywhere) as their relative proportion of highly able girls is larger.
However, as long as a school has a cohort within the year group that matches your child's ability the outcomes will likely be the same. Most schools have a range of abilities within them and the teaching is adapted accordingly to maximise the children's outcomes.
Therefore, going for the most selective school as your main criteria isn't necessary or even beneficial academically if it comes at the expense of other aspects of their holistic development and well-being.