Re: sub-launched nuclear armed cruise missiles
"1) Design and build new cruise missiles.
2) Design and build new nuclear warheads for the cruise missiles."
No comment on the sanity of nuclear cruise missiles, but...
The Royal Navy bought Tomahawk missiles in 1995, and all Navy subs are Tomahawk capable. And the Tomahawk could carry a nuclear warhead, though the US retired all nuclear Tomahawks in 2013 or earlier.
https://en.wikipedia.org/wiki/Tomahawk_(missile)