A deterrent cannot depend on an improvised deployment at the moment tensions are highest. Sending one or two ballistic-missile submarines to sea during a crisis would advertise escalation and would be difficult to conceal. A boat already on patrol is a survivable fact that an opponent must account for, while one left in port becomes an attractive first-strike target. Replacing the submarine system with air-launched weapons would also require Britain to abandon established submarine infrastructure and build a heavy-bomber force it did not possess.
Notable Point · 4:12-6:38
The logic of continuous at-sea deterrence
A British ballistic-missile submarine is most stabilizing when already hidden at sea; trying to deploy it during a crisis advertises escalation and leaves it vulnerable in port.