As stated in the 2010 strategic defence and security review:
“The Government will maintain a continuous submarine-based deterrent and begin the work of replacing its existing submarines.”
A deterrent works only if it is credible and available. All the evidence points to a continuous at-sea deterrent, based on Trident, as the best way to deliver the UK’s deterrent effect. A part-time deterrent—for example, where we do not have a submarine permanently on patrol—would make us vulnerable to a pre-emptive strike, and the act of deploying the deterrent in a period of tension would risk escalation at a potentially critical moment.