I would argue that current events in the last few years have overtaken this viewpoint. Specifically in the case of Russia, Iran, China and North Korea. Russia's invasion of the Ukraine has raised the spectre of state-on-state warfare in Russia, and their stockpile of 5,000 nuclear weapons increasingly modernises in line with the rest of their armed forces. Iran's nuclear program is still being negotiated, and they will probably gain a significant nuclear power program and lessened sanctions in exchange for their nuclear weapons program. This will reduce the lead time necessary to produce a nuclear weapon further, and will probably lead to demands of other middle eastern countries for similar programs.
This is a particularly dangerous situation, as these nations may also make a similar quick leap from power to weapons, potentially leading to a nuclear armed race, and in combination with their unstable, dictatorial nature and the rise of ISIL, we may well face an aggressive, expansionist, nuclear-armed Islamic dictatorships in the middle east.
China's aggressive action in the South China sea is underscored by the knowledge that their possession of nuclear weapons renders outright confrontation unlikely, even as the US pivots towards the pacific. They will no doubt continue their island grabbing campaign, safe in the knowledge that with nuclear weapons trained on Taiwan and Japan, the US will find it difficult to launch military action against them, but Britain's concerns lie with India, against whom they share a border and a history of conflict. With some 50% of Britain's trade due to be moving through the india ocean, we have significant commercial interest in seeing the diffusing of a possible Mexican stand off between indian, Pakistan and China.
Finally, North Korea's nuclear weapon program continues apace, with the recent launching of medium ranged ballistic missile, and the possible launching of submarine based systems. Future developments are likely to be based upon the creation of a true SSBN program, capable of targeting the US and her Allies, and as Britain attained a high profile in the Korean war, with our stand at Gloster hill and the Imjin, coupled with our support of the US, we would also be a likely target.
We must therefore reassess our strategic and defence capabilities realistically in the light of these developments. Your outlining of deterrence as depending upon a threat is only partially correct, as you neglect to mention that deterrence rests upon force, and the threat of force. according our enemies, but for the moment Russia, will judge the UK and NATO on this basis. Their previous invasion of Georgia and the Ukraine, coupled with their recent flights against the edge of UK airspace and the threatening of Nuclear attack against Denmark mark Russia as a state willing to utilise military force and nuclear blackmail in the pursuit of their geo-political objectives. This threat must be placed against our own situation. The extensive curtailing of our armed forces, Infighting in the EU on both internal and external policy and the poor way that NATO nations have committed to re-armament following the Wales Summit makes further aggressive action by Russia more, not less likely. At the moment, an article five violation against the Baltic states would most likely be greeted with submission-and a great fait accompli for Russia, with us accepting the loss of these nations right to national self-determination in favour of saying much but doing little, our inability to react underscored by Russian conventional and nuclear stength. This will shake NATO to its core, and provoke further attack.
These conclusions will also likely be arrived at by powers in the middle east and North Kore. We are in short faced by a number of unstable, expansionist, repressive nuclear armed or nuclear-developing states. Further disarmenent, especially of our nuclear weapons, on the basis that we face no creditable or suitable threats, is a naïve, uniformed point of view that is almost five years out of date.
The best means of combating this will depend upon action across a spectrum of capabilities-political, economic and yes, military, to provide the hard muscle crucial behind diplomatic soft power. What is critical is the rearmament of the UK, and that will include a creditable nuclear weapons program, for the medium to long term. In the Short term the renewal of trident should be coupled with military and political support to the Baltic states to deter further attack, for the medium term a carrot and stick approach in the middle east to deter a nuclear arm race, as well as long terms planning surrounding the India, China and Pakistan and our relations with these countries.
Further Reading
http://www.theguardian.com/commentisfree/2015/apr/20/putin-nuclear-trident-russia-britain-deterrent