The Prime Minister (PM) is the leader of the political party in power. He or she appoints the members of the Cabinet and has control over many important public appointments. The official home of the Prime Minister is 10 Downing Street, in central London, near the Houses of Parliament: he or she also has a country house not far from London called Chequers. The prime Minister can be changed if the MPs in the governing party decide to do so, or if he or she wishes to resign. More usually, the Prime Minister resigns when his or her party is defeated in a general election.
(Source: 2nd Edition, Life in the United Kingdom)