Fossils it's literally a "my dick is bigger than your dick" contest.
All Trump means is that he will find something to do if North Korea actually launch a warhead with malicious intent. He probably doesn't know what this is. He simply feels obliged to posture.
IF both countries actually do launch and IF both missiles find their targets? Massive loss of life. Asia and environs would suffer terribly, as would the US. For the UK? Nuclear winter, radiation to a lesser or greater extent. Think Hiroshima/Nagasaki, but on much larger scales.
However, please bear in mind that it is not Trump's decision to make. As a previous poster mentioned, Congress would have the final say.
We have been here before: The Cuban Missile Crisis was the closest the world has come to nuclear war and that was all a cocks on the table scenario. It didn't happen then, it's very unlikely to happen now.