Leaving aside the ballistic missile/cruise missile issue for a moment:
SDR (1998) said we need ten SSNs. SDR - new chapter (2004) said we need eight. That number seems to have dropped to seven. Now you want the SSNs to do the deterrent role as well normal SSN tasking?
See
this RUSI article (with link to a PDF format paper) on doing it more cheaply by modifying the CASD posture.
1. A 'Normally-CASD' Submarine Force - Extend the Vanguard-class submarines, delay the start of peak spending on the renewal programme until 2019/2020 and redefine what is meant by 'CASD' to cut the fleet of boats from four to three.
2. A 'CASD-Capable' Submarine Force - Abandon CASD in normal circumstances, but maintain a credible capability to reconstitute it if required. This option could cut the fleet of successor submarines from four to two and delay peak spending until 2023/2024.
3. A 'Dual-Capable' Submarine Force - Rationalise the submarine fleet around a single model of boat, which could be used either for conventional or deterrent roles. This new model would eventually replace both Vanguard and Astute class submarines.
4. A Non-Deployed Strategic Force - A more radical option, this would abandon the UK's submarine-based nuclear deterrent, maintaining only a non-deployed arsenal. Offering the most substantial financial savings, this option would still aim to provide a guaranteed - but not prompt - ability to retaliate against future nuclear attacks.