[What the threat levels mean
Threat levels are designed to give a broad indication of the likelihood of a terrorist attack.
LOW means an attack is highly unlikely
MODERATE means an attack is possible, but not likely
SUBSTANTIAL means an attack is likely
SEVERE means an attack is highly likely
CRITICAL means an attack is highly likely in the near future]
https://www.mi5.gov.uk/threat-levels
Current Level Today Number 3
I would put it up one level to Severe