I don’t live in a grammar school area but there is back door selection by affluence (one of best secondaries is in a nice leafy suburb) or by religious belief (equally high achieving secondaries are c of e or Muslim). I have no personal experience of them apart from the fact that my DF attended one in the 40s, enabling him to leave his deprived hometown and go to a fairly prestigious uni.
Many posters in the past have talked about sending their dc to private preps then trying for a state grammar at 11 which surely puts said children at a huge advantage due to smaller classes, better facilities and active preparation for the 11 plus.
How come the grammars don’t therefore give precedence to state school educated children who pass then allocate remaining places to those who weren't ? Or have a slightly lower cut off point for those children who attended schools in particularly deprived areas ? I appreciate that’s probably a fairly simplistic idea and prepare to be flamed 