Impiger,
I believe you are right to question if we need single role assets to deliver a nuclear capability, but I also believe that we absolutely DO need a nuclear option. Leaving aside the means of delivery, possession of a credible nuclear option gives the Irans and North Koreas of this world pause for thought should they contemplate any aggressive action against us, our interests and our allies.
As for delivery, a submarine does have that handy capability to creep around the globe undetected. Not too many other ways of achieving global reach for a nation like ours. Carriers are all very well, but they are vulnerable without the full monty of screening vessels and aircraft, and that makes them pretty expensive.
Its just my opinion of course, and our well beloved government will have our best interests at heart. I'll rest easy in my bed tonight knowing just how many impressive ministers will be considering our nuclear defence option.