Based on their historical data, it seems to happen 2-4 times per year on average. Obviously the situation is very tense right now everywhere in the world but this isn't exactly exceptional.
A simple example would be cancelling badge access for a janitor to a sensitive area like an engine room, but allowing it if the threat level Was lower.
https://en.wikipedia.org/wiki/UK_Threat_Levels
Seems to change every year or so. It was at severe a year ago, but was reduced to substantial, now it's back at severe.