The program was thought provoking I agree, as I understood the end of the episode, there was a possibility that the Russians were about to launch a full scale nuclear strike on the UK, only a possibility.
The UK had at least two Vanguard class subs deployed at 15 minutes readiness to fire, each of those subs had the Prime Minister's letter of last resort in its safe.
The fact that at least two Vanguards were at sea would have been known to the Russians, taking that leap of faith that the Russian leadership under Putin is logical, Russia launching a nuclear strike would be a gamble on the the decision making processes of the UK's prime minister, that would become apparent if the UK is targeted when the letters of last resort are actioned.
So overall a logical decision.