The way if works is that the parliament is elected for a fixed term, and they appoint a head of government from their own ranks. The head of government can dissolve the parliament ahead of time, and they can also resign or be voted out with a motion of no confidence, in which case a new head of government needs to be appointed. If the parliament cannot or will not appoint a new headof government there is a new election.

As Labour has an absolute majority of the House of Commons, Brown's appointment should be a formality. He's widely expected to call an election next year, one year earlier than expected and two years ahead of the end of the 5-year term.

In more proportional systems with multiple parties and coalition governments if the government fails usually a newcoalition cannot be assembled and a new election is called. But this has not been the case with Balkenende in the Netherlands during the previous term. He had several cabinets with changing coalitions without having to call an election because he was always able to assemble a new coalition. Also, recall the Czech parliament took several months to get a prime minister who could pass an investiture vote because the parliament was split exactly 50/50.

It's not like the US constitution, but it's not wrong.

Can the last politician to go out the revolving door please turn the lights off?