Generally the government in the UK lasts for its whole political term and so does the prime minister, the conservative party (before the current government took power) experimented though with a series of short term prime ministers. Think like a disposable camera, but a prime minister version.

On the one hand, it's really useful and interesting to see how different ways of phrasing the question bias my intuition, but on the other hand, it's a real pain in the neck trying to arbitrage all the various versions! Somebody needs to write a bot!