The UK has made public its previously classified National Risk Register.
The National Risk Register is intended to capture the range of emergencies that might have a major impact on all, or significant parts of, the UK. It provides a national picture of the risks we face, and is designed to complement Community Risk Registers, already produced and published locally by emergency planners. The driver for this work is the Civil Contingencies Act 2004, which also defines what we mean by emergencies, and what responsibilities are placed on emergency responders in order to prepare for them. Further information about the Act can be found on the UK Resilience website.
Seems like the greatest threat to national security is a flu pandemic.