You can apply the parameters of today and extrapolate to get a picture of the future. Advanced democracies will not go to war with each other, they share a culture, interests, they're interconnected and reliant on each other. There will probably never be a war between them because they keep getting closer with each passing decade and borders are becoming less relevant or they even dissapear (the EU or Canada/US). The trend is clear, these countries will eventually merge into a single entity and, in the long term, war between them is as likely as war between Essex and Aberdeen.
Furthermore, there's no alternative to the liberal democracy that leads to the same levels of efficiency, innovation and prosperity. In other words, countries such as China or Russia will either transition into democracies or forever play catch up in terms of technology. And since technology dictates who holds the biggest stick in the modern world, those coutries will never be a true threat.
As for massive disasters, you watch too many fantasy films. The natural ones are extremely unlikely and the financial one we've been through has been harsher on the rest of the world compared to the US.
So again I ask, what threats are dettered by British/French/etc nuclear weapons, as long as one democractic superpower has enough of them?