Basing SSBN and Refitting them at Plymouth are 2 very different things. The current boats can only get into and out of Devonport in the very light condition (ie just prior to, and on completion of being refitted) and with certain heights of tide. It is not viable to base and operate the SSBN force there permanently unless they were very much smaller vessels.
Given that we are committed to retain the Trident system, and that defines the diameter of the submarine hull I don't see that as being an option.