The biggest problem a nuclear deterrent has is that it must be invulnerable to a first strike ensuring that no matter what is fired at us, we can still fire back.
The biggest disadvantage of making it airlaunched is that its fairly easy to take out an airbase with a big bomb - we know where it is, we know roughly whats there and hopefully a big bucket of sunshine will do the trick.
The big advantage SSBNS offer is that they are almost silent (allegedly) hard to track and with probably 3 at sea in a REALLY bad crisis very difficult to take out all at once (assuming the UK decides that if one goes we may well fire).
My preferred view option would to to "nuclearise" the storm shadow AND the TLAM or its succesor, providing a cheap and cheerful deterrent which can be at sea and land/air simaltaneously. The challenge this presents to an aggressor is to have an armed force capable of doing ASW AND land attack at the same time with nukes.
Of course we could all learn to love each other...
