They measure as the crow flies from the door of the school to your front door. Outside the area (which might be very small if the school mostly collects the lion’s share of it’s input from another feeder school), you stand very little chance if a school is over subscribed.
Siblings of kids already there and special needs get priority regardless of where they live.
The government forces councils to hold a kangaroo court which is called “appeal”. For popular schools, it’s pointless and rather demoralizing, – but they are not allowed to tell you that. I’ve done it. All parents have a right to appeal though, whatever the (more or less pre-determined) outcome.