Who has actually been stopped because of our deterrent? North Korea is still having fun playing with missiles, various countries still using chemical weapons, not a single nuke has been launched. No one wants to make that call, chances are, no one will make that call, a nuclear weapon is going to kill a lot of innocent people.
The world should decommission all nuclear warheads, but that won't happen. The UK keeping them however seems a little pointless, it's a waste of money and if we aren't going to use them to strike first, they are a bit useless. Use the money in more important areas, like health.