Are you suggesting we would engage in a nuclear strike if nation X invaded us with a conventional army.
I don't believe remotely this is the case.
Personally, I think there are too many assumptions here.
The real threat of nuclear weapons is global instability, war & the fact so many nations already posses them.
The NPT had an aim for eventual disarmament, something which angers at the moment the members who have not taken steps to gain there own. There is also the argument that while nation X has nuclear weapons, this promotes proliferation in nation Y to compete.
If we want to play far flung & paranoid made up scenarios such as Russia invading I could make one up which is equally believable with the direct opposite conclusion.
Besides, a base deterrent could be maintained with a number of hidden remote silos at a much lower cost than trident.
https://www.gov.uk/government/uploa...12745/20130716_Trident_Alternatives_Study.pdf