They didn't choose it, in the end. My GS isn't due to start school until this summer. They went to an open day and were told by staff that there'd be no point in them applying because GS didn't fit any of the criteria except it was their nearest school. My DIL isn't white, British or Christian and she felt very unwelcomed by their attitude, that her face didn't fit.
I live nearly 600 miles away so I don't know the ins and outs of it all but I think places go to children of people who belong to the church, those who attend other CoE churches in the area, siblings, cared-for children (though there will be vanishingly few of those because there's a lot of money in the area) and who live closer to the school. People want the school because it has good reports and provides out of hours care but thankfully so do other schools in the area and they've found somewhere else for him, further away.