It really depends on the circumstances, I don't think a nuclear response is guaranteed. As an example, in a situation where the CandC structure is completely decimated I believe individual sub captains have the final say on whether or not to retaliate, I'm not sure how many people would sanction the strike given that we would have already been effectively wiped out, what's the point?
Equally if we're talking a single strategic strike, is there a benefit to retaliatory launches if the likely response is all out Armageddon? Could it be better to take the hit? You could argue that it undermines the deterrent but what use is a deterrent when you're dead?
It's an interesting thing really, we have a weapons platform capable of unleashing untold destruction but it becomes completely useless as soon as the criteria for its use are fulfilled. If there are 1000 missiles in the air what's the point of throwing up ours?
I'm not saying we wouldn't launch, I'm just saying it's not clear cut, it would take a more stoic (and possible stubbornly pigheaded) person than myself to be able to make that call.