Glenn Rhoads:
They don't. They elect Members of Parliament. The prime minister is nominated by the monarch and Parliament ratifies ('vote confidence in'

her choice. Of course, the nomination is a formality, because the only person whose nomination Parliament would ratify would be the leader of the party able to command a majority of votes.
The PM doesn't even have to be an MP, although a PM who was not an MP would be expected to try to become one at the earliest opportunity. This could happen if the party leadership changed between elections or if one party won a majority in the general election but its leader did not win his/her own seat. In Canada, where we have the same system, the former case happened in 1984 when Pierre Trudeau retired as PM and the party chose John Turner, who was not an MP, to replace him. Turner promptly called a general election and did win a seat for himself, but his party was defeated and he in turn resigned as leader afterwards.
Dan Tilque:
The 5-year period runs from the last general election; a change of PM doesn't affect it. In Canada essentially the same rule is a consti- tutional requirement, and I thought it was in Britain too (although the term doesn't have exactly the same meaning as in Canada or the US).