Why do we need Trident II?

One of the many policies of the government that I applaud is its decision to review the Trident II policy, instead considering whether alternative nuclear options might be preferable.  Much of the debate concerning Trident frames the discussion in terms of its cost, and when one's budget deficit is as high as ours, I guess that's inevitable.  But for me it's not about cost, it's about our strategic goals.

The MoD clearly doesn't regard Trident II as part of the UK's defence, since it didn't want it paid for out of the Defence budget.  Repeatedly, in the argument about who should fund Trident II, it was insisted by Defence bureaucrats that Trident II is a "political" project and should therefore be funded by the Treasury.

Why do we need a continuous-at-sea multi-megaton nuclear deterrent?  The only countries that seem to me potentially deterred by this are the French and the American.  But I'm guessing we'd get a bit of notice (say, about twenty years) if relations with these, our closest allies, were deteriorating to the point at which they were planning strategic nuclear strikes on our major cities.  And with the benefit of this extra notice, one might imagine we would have the time to produce something additional - we wouldn't simply be stuck with today's deterrence capability.

Failing, that, who else?  In the Cold War we used to deter the presumed conventional weapons dominance of the Warsaw Pact with the threat of nuclear retaliation.  But I have a feeling that the Czechoslovakian army isn't going to be storming Dover any time soon.  Is the idea, perhaps, that we might fancy invading Russia and would want to deter their nuclear defence retaliation?  I'm guessing not.

We certainly aren't deterring Iran or North Korea.  They don't believe for one moment that we would respond to their delivering a nuclear weapon to Iraq or South Korea by our hitting them with Trident.  Quite apart from the disproportionality, such a response would be absurd, since we could utterly destroy them with unleashed conventional forces anyway, and with much greater precision.

"Ah", I hear someone say, "but new strategic threats may arise in the future that we can't anticipate now."  So what?  That only matters if those new threats arise so quickly and with such power that we are unable to change tack ourselves to respond.  And how would that work?  Why would our new enemies be so much quicker at acquiring a strategic threat to us than we would be at producing a new strategic threat to them?

Bear in mind that I am not, for one moment here, suggesting we should give up having nuclear weapons.  That shouldn't even be up for discussion.  Britain is a nuclear power and should remain so.  But do we currently (or, say, for the next decade or two) need a continuous-at-sea multi-megaton system for our nuclear threat?  I can't see what the case for that is supposed to be.