Jacko and Althenick,
Basic nuclear deterrence theory I'm afraid. An SLBM force is the only true "second strike platform". Doesn't matter how stealthy aircraft are if they are nuked on the ground. The only platform which comes close to the SLBM for survivability is an uber hardened silo based ICBM. Long ago the UK opted for SLBM as giving the best nuclear bang for the buck, whilst maintaining a coherent nuclear deterrence strategy.
There are many books on the subject, two of the best being "Deterrence" and "The Evolution of Nuclear Strategy" both by Lawrence Freedman.
I agree that deterring a rogue state which has a small nuclear capability with Trident is massive overkill, but that is not what Trident is for (despite what a politician might say). Trident is part of the deterrence against the major nuclear powers, Russia and China, and therefore de facto provides a deterrence against all sorts of other scenarios.