Most planning in the UK, is a dismal failure, leading to horrible results. On the other hand, the building regulation control is reasonably effective at producing safe buildings.
I suspect that is a matter of opinion. As I see it, sometimes it is good, and other times bad, with some of the planning decisions.
I suspect, that as the UK is relatively crowded, it makes strict/careful planning control, more important. Than somewhere with lots of wide-open space, and plenty of room to build stuff, without affecting others.
Over building in places in the the UK, could cause terrible traffic situations, badly over-load services, such as Doctors surgeries, Hospitals, Schools and other facilities. Make the place over-crowded, possibly crime ridden, and perhaps make it horrible, for most people, living in that area. Not to also mention, potentially increases in pollution, littering, street-crime, traffic-queues, road accidents, and many other nasties.