Definately, we should definately have a nuclear arm...
Now...I'm not a major fan of nukes...the whole thought of the buggers gives me the shivers, but...we shouldn't have rely on others for our strategic deterrant...we are a small island nation, we can defend ourselves conventially against conventional force...but aside from ABMs (which we don't yet have) the only way to prevent nuclear destruction is the capability to launch a retalliation on the country which launched on you....MAD...Mutually Assured Destruction.
Admittedly, having ICBMs isn't going to prevent a CBN attack by Bin and co, who are Britains main and closest threat at the moment (well...unless you count Brussells or Westminster
) as nuclear weapons are a strategic weapon, not particularly a tactical one (although Tac nukes do exist, obviously...but not the ICBM types....and I'm not 100% sure the UK has Tactical nukes...I think they went the same way as the Vulcan...)