This scenario played out a lot under COVID where TAG and CAG grades didn’t have the usual spread of results. As above, everyone who firmed and met their offer in full had to be accepted, (but it also meant a lot of very near misses who normally have been accepted were rejected, eg. A*A*BB wouldn’t have been enough for an A*AA offer).
Some unis offered incentives for students to defer, but that was unusual.
Unis are good at balancing their offer volumes and will try to oversubscribe a little to balance out natural attrition.