Ah yes, agreed. As said, I became a secondary teacher/6th Form specialist and my kids went to school, in Leics which was fully Comprehensive. Well not fully, especially in my last school, as it was a small University town with 3 secondary schools with 6th Forms- so the creaming effect was very much there. Ours was Comprehensive but there was a Catholic School across the road (and I know that many who attended were 'Catholic' only due to grandparents- and only attended Church so that kids could access the school) - and a private school with a long tradition.
Our school had a huge mixture- and thank goodness, some very bright students from academic parents from the Uni across the road. The school would have been much better without the 'creaming' effect. So yes, I can see how, where Grammar schools are still present in the State Sector, that would be even more so.