Violet Club,
Given your handle I wasn't expecting that reply!
Your arguement has much merit; nuclear tipped TLAM-N from SSN, S-Shadow from Land or Sea Based air are both plausible options. A few small problems though.
1. SSBNs, like carriers, are political viagra at the top table (ie UN). Few politicians like giving up any form of power..
2. IIRC the Trident boat replacement is as much to do with maintaining Submarine production in this country as it is with maintaining the deterrent. If new SSBNs were not built, there would be a huge gap between SSN classes. We could just build more, more useful, SSNs though..
3. TLAM-N / S-Shadow might be vulnerable to a sophisticated IADS in the future; a potential enemy would have to invest mega-bucks for Ballistic Missile defence.
4. The boats are home-ported in Scotland....