Firstly to set the scene. Suppose a rogue nation managed to obtain a nuclear weapon, lets say; stolen from a poorly protected stock pile in another country. They only have this single nuclear weapon and have no nuclear program of their own.
They detonate this acquired nuclear weapon in a major UK city.
Would you want our country to retaliate in kind and detonate a nuclear weapon killing ten of thousands of innocent people?
They detonate this acquired nuclear weapon in a major UK city.
Would you want our country to retaliate in kind and detonate a nuclear weapon killing ten of thousands of innocent people?

***gets coat and shotgun***