Much of the rationale for the SSBN based replacement to the V-Class is the simple fact that without it the UK will be out of the Nuclear shipbuilding business. The gap between the last of the Astutes and the first of their replacement would be simply too long (and thus expensive) to keep a highly skilled workforce in mothballs. Hence why we're talking of replacing the subs and re-working the missiles.
FWIW the debate needs to be do we need this indigenous capability. If we do then let's build the new Bombers as it remains the best way to guarantee a second-strike capability, If the answers "no" then we would probably have to go back to a mix of TLAM-N and Storm Shadow-N....not exactly cheap either.