Number of warheads?
So if we cancel Trident and move the warheads to smaller, MUCH slower and shorter-range, yet-to-be-developed cruise missiles that are more likely to have countermeasures work successfully against them than an ICBM with multiple warheads, do we then anticipate that we need far more of them in order for some to get through? Precision weapons that can find a particular window x,xxxKm away and fly through said window will still leave a lot of nasty stuff lying around if they have their wings clipped by something before they get to the target.
This begs the main question - Are we prepared to have the right number of launch platforms in place at the right time. In other words, we'll need more long-range submarines on semi-permanent patrol close in to shore-lines, or fleets of aircraft 'somewhere' in the world ready to fly (and presumably run the risk of crashing or getting shot down), and the backup facilities of those nasty things called runways and hangers/engineers/fuel dumps. We could take it with us as an 'Aircraft Carrier', but how many countries would let it dock?
I believe the NPT also has very strict rules about developing new warheads as well as controlling numbers, so we can't just pinch them off Trident and super-glue them to the sharp end of a cruise, or can we?
I think Trident and it's predecessors work, although I'm horrified that the world is such a place that we need it. I don't want to spend the money replacing it but it won't last forever and we know we really need four of them for it to work properly and not stress the crews or equipment too far.
Some clever people are going to have to come up with something much more credible rather than LibDem wet dreams.