There is definitely a risk that this proposal would have the effect of making school black spots completely insurmountable, particularly for BTC families who didn't get into one of the HW or BV duo. I'm not BTC, but I am pretty adamantly opposed to these proposals as I don't think it's a great idea to do this.
What the policy is designed to do is make families use local schools when they move. But there is no guarantee that the older children would get a place in the local school - so people will end up stuck trying to do two or more drop offs at different places anyway.
It's also a nightmare for families who through a move or simple catchment shrinkage now find themselves more than 800m away. Those families made relied on the policy and may now find themselves stuck with no school options. This would particularly be a problem for low income or housing association families who don't often get a huge say in where they live and could be hugely disruptive to children who are potentially most vulnerable.
Cracking down on short term renters was a good start, but I think these proposals will make the problems worse.
Although they will likely make real estate agents selling houses within 800m of a good school very happy. Just what we need