SSBNs are a symbolic luxury for British forces in this day and age. The cost of developing and deploying (not to mention disposing of) these single-use submarines is simply not affordable. Why not spread the Nuclear issue around? Nuke SS shouldn't have been prohibitively expensive (and would have provided job security for AWE et al) and TLAM-N is already a mature technology - what's wrong with packing a couple of TLAM-Ns into an SSNs VLS? Plus, a combination of delivery systems enables you to make any potential enemy guess from where the threat is coming from. Is it from the SS equipped CVF CAG? The SSN? The Nimrod MRA4? The implied threat of using a nuke is massively enhanced by actually declaring bit of your hand; deploying a Sqn of nuke-capable TypHoons is a "diplomatic" message in itself. I think we're going SSBNs because of political pressure to keep Sub yards in business between SSN classes. This is not a bad aspiration, why not just come clean about it?
Or we could just contract out Nuclear Deterence to go with MFTS!