What would be the use in building 5 submarines instead of 4? The purchases of four submarines were made because that many submarines are needed to maintain one on station at all times. What does that fifth submarine get you in terms of deterrent capability?5 Resolution class SSBN were built instead of 4. The fifth boat was ordered in 1963 along with the other four boats. It was laid down in 1965 at Vickers, Barrow or Cammel Laird and completed before the end of 1969. Furthermore, all five boats were refitted with Poseidon immediately after the USN finishes rearming its SSBNs with Poseidon. Consideration was given to rearming them with Trident 1, but it was decided that Poseidon would be adequate until the Vanguard class entered service.
5 Vanguard class SSBN were built to replace the Resolution class. They were also built at a faster rate than the OTL boats. IOTL the 4 boats were laid down 1986-93 and completed 1993-99. ITTL the 5 boats were laid down 1986-90 at a rate of one per year and completed 1993-97 at an average rate of one per year.