The problem with a pierside SSBN alert

Could Britain keep two SSBNs and deploy one from the pier only when a crisis required it?

No: a pierside boat is both vulnerable and difficult to deploy without escalating a crisis, so it does not provide the same deterrent as a continuous patrol.

The answer returns to the distinction between a standing at-sea deterrent and a boat waiting to deploy. A submarine held in port is vulnerable and its sortie during a crisis can be read as an escalatory signal. A four-boat force is discussed as the practical basis for guaranteeing one submarine on patrol despite maintenance and training cycles. The objection to a two-boat pierside plan is therefore about survivability and crisis stability, not simply the number of hulls.