I've been mulling over this and don't like it. This is because I, like Balhamite, am in a bit of a schooling black hole. My eldest has got into a school more than 800m away. Some years we would have had no chance, but this year we were lucky and I found out that we were the last to get in. Why should my kids have it go to different schools just because we were lucky one year? It would only be fair if the catchment for ALL pupils, not only siblings, were 800m, then it would be consistent. But that wouldn't work for the council as the school places have to be filled.
I quite like the concept Gruffalo's Dad is proposing. Something like that would address my concern.
The thing is, also, for people moving into an area and hoping to get their kids into a school with tiny catchments of no more than around 200m, that information is there upfront and they have to work around it. What if you don't have the resources to move to get into the school you like the look of? The proposed policy is unfair to people who stay in their current house/flat for years, for whatever reason. You shouldn't be forced to move because of a school. It is ridiculous.