No, although they can be used that way, that is not the purpose of them. The idea is that once out at sea, no-one knows where they are, so they cannot be targetted to be knocked out before an attack on their home country and will remain available to respond.
Indeed, they may not even respond - each Prime Minister gives a letter to the sub commanders, to be opened *after* an attack on the UK. That letter tells them whether to fire back or not in such an event. The idea being that an enemy won't know whether we would respond or not, but the PM has that control.
But subs hide much better. Time taken to arrive is not a problem for a retaliatory weapon though.
Agreed.
Although not used much, there have been commercial nuclear ships before
- and a number of icebreakers.