Mystery when the Grammar school system was fully set up there were usually two or three secondary schools in any one town, village children expected to travel the nearest town. DC would then sit a test and be allocated the appropriate school for their ability. They didn't choose to go to a Grammar school, it was the school they were allocated. Transport funding was then based on how far from your allocated school you lived. In my Grammar school most of the DD came from 'working class' homes and many of the DDs would have been entitled to free school meals. It was not a middle class enclave, most of the ones I am still in contact with are still politically socialist.
It was not an exact science just as now. Some children were allocated Grammar who found it too academic, it happens even for the untutored as most were then, some were allocated secondary modern when they could easily cope with Grammar. They often had the opportunity to move at a later date, some chose to stay in their secondary moderns. Parents also appealed but I believe it was rarer then and possibly less transparent than today. (no idea if stats exist but my parents went through this process). Some parents opted to go private if their children weren't allocated a Grammar, most didn't in my area as those that wanted private education didn't tend to sit the test anyway.
So if you live in a selective county you wouldn't be making a choice but sending your DC to the school that the county has deemed suitable for their academic ability. That is why I personally disagree with Bucks's current transport policy that will only pay costs to the nearest school. It makes a mockery of the whole idea behind selective schools.
I have no problem with people moving if they are unhappy with their local school but I do have a problem with people hedging their bets and by doing so taking opportunities away from other DC especially when it's DC who won't have the parental support that makes the difference when faced with a poorly performing school.
p.s realise I have spelled tier wrong previously and really can't be asked to go back editing, no wonder my DC have dyslexia