The United Kingdom has maintained its Continuous At-Sea Deterrence policy, known as Operation Relentless, without interruption since April 1969 by keeping at least one nuclear-armed submarine on patrol.

From the articleThree of UK's four nuclear submarines are out at sea in ‘unusual’ move: ReportAlso in Three of UK's four nuclear submarines out at sea, Why are three of the UK’s four nuclear deterrent submarines at sea?, Why the deployment of 3 UK nuclear submarines is raising eyebrows