Nah, Sorry, but you really haven't justified it.
I still don't see that in order to have a seat at the top table (if that seat is ours only by dint of having nukes) we need to have an SLBM based nuclear capability.
I don't see real evidence or proof of the claim that ONLY an SLBM can deter Russia or China.
Nor even that only an SLBM gives a second strike capability, though that's a less contentious claim.
There's certainly no evidence that Russia and China are the threats that our nukes need to deter, nor that we need a second strike capability. This isn't 1974, you know.
And you certainly haven't justified the claim that we'd "end up with a capability well short of Trident, without the same relative decrease in cost."
You may "think many countries would be of the view that either we replace Trident like for like, or we step back and unilaterally disarm our nuclear capability", but where's the evidence for such a view?
Your arguments seem to me to be Cold War thinking at its most stagnant and dusty, with a healthy dose of single service Matelot tunnel vision thrown in for good measure.