NotSpaghetti, my knowledge, which I acknowledge will be out of date, now, twelve years into retirement, refers to the county of my schooling, and a large part of my teaching career, Kent.
Kent has a selective secondary system, and had, and continues to have, single sex grammar schools.
The selection criteria for both boys and girls was not based on the pupil's neighborhood, nor the ability of the parents to pay fees, but on a series of tests .
There was, of course, some manipulation to ensure that the number of successful boys equalled the number of successful girls, but that would certainly be a subject for a different thread!