I've posted my thoughts elsewhere - in brief, replacing trident is lunacy on cost grounds, £2bn a year for 20 years on something we'll likely never use, don't want to ever use and that can't do anything else in the meantime.
What would make sense is keeping building smaller tactical weapons (maintains the AWE skills to build warheads) which can arm cruise missiles (clones of US W80 warheads basically). That means we can use Astute class attack submarines in a limited deterrent role (yes, very limited, but still a deterrent). So the navy might get some more Astute class boats, which are actually useful for other things and we save some money.
A full blown ballistic missile defence with continuous at sea patrols is a bit of a luxury in my mind seeing as the cold war ended 20 years ago.