That's not how it works. The US
also relies on the ability to promptly respond to a first strike, which say the UK does not.
What I'm pointing out is that you've conflated two things. Just because one aspect of your deterrence is the ability to launch your land-based ICBMs within 30 minutes, that doesn't mean that the inability to do so (e.g. because you don't know who struck you) means that you're out of options.
Besides, there's no way someone could amass enough SLBMs in the Pacific to take out two aspects of the US's nuclear triad without the US knowing who owns those submarines.