Tradition does dictate that the PM is an MP, and tradition in UK parliament is pretty binding. However, an MP does not even need to be a British citizen: an Irish or a Commonwealth citizen can become an MP and go on to command the confidence of the house.

> Tradition does dictate that the PM is an MP, and tradition in UK parliament is pretty binding.

I hope it stays that way.

The US had pretty strong traditions in politics, too, but they are increasingly being eroded.

To give an example that's hopefully far enough in the past to be non-controversial: when they banned alcohol in the early 1920s, they felt that their constitution did not already explicitly give their federal government the power to do so. So they passed the 18th amendment to give the feds that power to specifically ban alcohol.

Decades later, in the context of the war on drugs, everybody seems to take it for granted that the federal government can obviously ban arbitrary substances.