Very few laws in the UK requiring a prime minister either. That said, there is one missing power which is critically important: the Queen acts by convention on

Very few laws in the UK requiring a prime minister either. That said, there is one missing power which is critically important: the Queen acts by convention on the advice of her ministers, and by convention that is the advice of her prime minister. So although there's very little legal power to the office of PM in either Canada or the UK, by effectively controlling the use of the Crown's prerogative powers its a very powerful office indeed. Of course, the convention could change, and the Queen take someone else's advice. But that would be a major constitutional change that would make today's relatively small development pale into insignificance.