Since the Tories ( who are sadly likely to get the next go, and I'm not a Labour fan either ) have already stated they will make 25% defence cuts; that is a HUGE cut !
In a perfect world - sort of - I'd agree with the Trident replacement, but if we do that aren't we certain to lose the carriers and / or JSF ?
My main worry is China, the nutter middle east states wouldn't care about being wiped out, so little deterrent effect.
My suggestion would be to bin the Trident, have more Astutes with nuclear Tomahawks ( yes they can be shot down but who's going to gamble on getting all of them ? ), build a sensible no. of Type 45's ( I believe the PAAMS missile can be used for point anti-missile work with a bit of tweaking, as the Aegis & Arleigh Burke American jobs can ), and build 3 more CVS style carriers flying Harrier 2+ with AMRAAM.
If cutting 25% - or more, we cannot afford Trident, the big carriers or JSF unless every useful bit of kit, ie the stuff that might actually get used, is forgotten.