I disagree with lewis
I agree we need a nuclear deterrent, and clearly it must be effective which does rule out relying on cruise.
However we don't need a like for like deterrent based on the one we needed during the cold war.
A pair of ballistic missile subs would do the trick for deterrence against the big players such as the US, Russia, France etc. and a few cruise chuckers would be fine against those countries without a decent air defence system like Iran, North Korea, Wales etc. They could also be used to lob conventionally armed cruise around should the need arise.
Look at it this way, currently we (the UK) have enough nukes to effectively destroy the world many times over, add that to the effect of any retaliations means we have plenty enough boom-boom and then some.
But the real question is: how much do we really need? Remember the whole point of nukes is as a deterrent. We don't have them so that we can destroy the world we have them so that everyone else is afraid to set them off. They don't call it MAD for nothing. Chuck Norris and Ninja Steve Jobs would have to be polite and diplomatic with each other because no-one could afford the consequences if it turned violent - and it is exactly the same with nukes (except nukes are obviously not as dangerous as Chuck and Steve-san).
So what do we need? Well with respects to the lunatic fringes cruise should be fine, failing that a couple of ICBMs would do the job all peachy like. So what about the big boys, such as the USA or Russia?
The real question would be, what level of destruction would it become untenable for any US president or Vladimir Putin to be able to sustain? One city? A couple of cities? Disneyland Florida?
Joking aside, you would only need to be able to guarantee a dozen or so warheads to present an effective deterrent. Which would be a lot cheaper t buy and run than the current proposals.