I couple of years ago I heard a well made program on Radio 4 all about this issue. They were interviewing various senior officers and a whole bunch of cross party politicians who were on the committee that decided to go for the SSBNs alone. Interestingly, and surprisingly to me, they said the numbers were very clear that the SSBN option was the cheaper and more effective solution, even leaving aside the second strike issues with land based options. I can't remember the name of the program, sorry, but it was very enlightening.
Exactly the points being discussed here were brought up and the reason explained.