Having a Trident replacement is a national 'insurance policy'. Just like any insurance policy, its overly expensive, you never use it.....until that one time you might do....or at least be happy its there.....
The GBP have not a lot of strategic inclination, and most I would think wouldn't have a clue about the capabilities of any potential enemy. I wonder how many of them realise that Russia has stand off nuclear capable ALCM's that could turn the UK into a smoking hole before QRA got anywhere near them (as I understand it anyway).....
The fact they won't do it (well one of the considerations I'm sure) is that big boat underneath the North Atlantic somewhere with our own deterrent on board.....MAD....
On a separate note, RE The Labour Party, the biggest mistake they ever made was electing the wrong Milliband-its all gone REALLY pear shaped since-and its still heading South with JC.