It's worth noting that the level has literally never been below substantial or above severe.
How so? Looking at charts like [1] or [2] it seems current threat levels should be much lower than in the time before 2001. Notable terror attacks in the UK have gone from a regular occurance to a once-in-a-decade deal.
1: https://qph.fs.quoracdn.net/main-qimg-6aa8066136b8b875fc4805...
2: https://cdn.newsapi.com.au/image/v1/eca0e3ae8792b445038a7dd2...
While there were three figures, NI-related in GB did reach moderate.