Does the UK need to retain it's nuclear deterrent? & if so, in what form should that deterrent be?

Last night on the second election debate the parties basically said the following…..
Labour want a like for like replacement to Trident.
Conservative want a like for like replacement for Trident.
Liberal Democrats want to hold a review to determine what type of nuclear deterrent the UK needs, but the assumption is its not a like for like replacement for Trident.