More importantly, what messages is he sending out to Putin, Kim Jong-Um or even to ISIS*. If they think he wouldn't press a reliation button, that's all is required to render our entire nuclear deterrent impotent and worthless.
Whether a leader is *actually* prepared to do it or not, is not really the point. It's the message they send out that's important, and in that regard, he fails completely.
* Is it beyond the realms of possibility that ISIS would try to kill millions of Britains by putting Polonium in the Thames water supply, or some other such catastrophe? I am not sure it isn't.