Firstly, thanks again to Doubledaddy and JG75 for bringing this to my attention.
FWIW, I think the general principle of the proposal is sensible - ie allow for families to move to a larger/cheaper house as their family grows, but ensure that children go to their local - ie walking distance - school, and reduce the options for abusing the system.
But I do think 2 points of detail should be looked at. Firstly, 800m seems very tight. 1200m would seem more sensible to me. That's a 15 minute (adult) walk which an older sibling could certainly manage on a scooter. Local kids should be walking to local schools - it's healthier, less traffic, safer for all etc.
Also I agree with other posters who say that if the family moves away, but the original school is STILL their closest school, by walking distance, they should be able to appeal.
And also that if they haven't actually moved house (ie the catchment was wider than 800m - or my suggested 1200m - when they accepted a place for Child 1, they should be allowed to send Child 2 regardless).
But yes, will fill out the form, as this affects us all. Everyone wants a fair solution and for exceptions to be looked at, i think!