I am getting the sense that there is a difference on this point in / outside of London.
Where I live (not London), and I believe in many other LAs outside London, catchment areas are a real thing, and they work independently from distance from the school.
A catchment area is a red line on a map that has been set as the area that that school has been built and designed to serve (eg as part of the planning permission process for a new school). The aim - although not always achieved - is that all children within the red line should in theory be able to secure a place, and only after that has occurred will any remaining places be offered up to children who live outside the red line.
The extent to which children outside the red line will then be able to secure a place will depend on the admissions criteria for the school in question, and may or may not be linked to distance from the school (tho distance is often used as the ultimate tiebreaker).
Even within the red line, if the school is oversubscribed and there is not enough room for all the children within the catchment area, distance may not be the deciding factor. Again, it will depend on the school’s admissions criteria, but it is very common for other factors such as siblings, feeder schools, etc, to trump distance in these circumstances.