So the sugestionis is that Theresa May was lying when she said that she would be willing to kill hundreds of thousands in a nuclear strike?

Well, it could be true. It's all bluff, based on the hope that opponents believe our leaders are maniacs.

If a nuclear deterrent fails to deter an attack, there is no conceivable benefit in actually using the weapons. To actually launch the weapons would be completely without any value. It would also of course be an inconceivably evil thing to do, but perhaps that is not something that influences most people in positions of power.