Why Britain did not build a fifth Resolution-class SSBN

What was the logic behind the proposed fifth Resolution-class SSBN, and what were its knock-on effects?

It was insurance for continuous deterrent patrols, but four boats proved adequate and the money instead supported two more SSNs.

Five ballistic-missile submarines might normally have placed two on patrol and made one continuously available with greater maintenance margin. The proposal was therefore an availability hedge rather than a demand for five boats on permanent operational duty. Once four Resolution-class boats demonstrated that continuous deterrent patrols could be sustained, the extra insurance looked less necessary. The procurement choice then favored two additional nuclear attack submarines instead. This exchange shows how apparently small changes in fleet size can express a deliberate trade between deterrent resilience and conventional undersea strength.