They were several reasons for building the Upholders.
During the 1970s it was clear that SSKs were quieter than nuclear-powered submarines, both Soviet and Western, for example during Exercise Teamwork in 1976 none of the four SSKs operating in the Shetland-Faroes gap were detected.
Also by 1977 it was becoming clear that the RN lacked the resources to support as many SSNs as it needed, so the cheaper SSKs (planned cost £52mil - 1/3 price of an SSN) could fill in numbers but with no loss of effectiveness.
It was designed for the Channel and Eastern Atlantic areas (especially the Iceland-Faroes gap) and would be used for surveillance and ASW training, a task not really suited to expensive SSNs needed elsewhere on patrol.
It downfall was the increased costs due to a host of problems with the torpedo tubes and the engines (originally designed for locomotives!) and this delayed the class. It also lacked range so if they were sent to the Mediterranean, they needed refuelling when they arrived. So with plans to drastically cut the submarine fleet anyway in 1990 after the fall of the USSR, the Upholder programme was stopped. But they were very effective submarines, just rather specialised and perhaps a little too cutting edge.
