Prime Minister of the United Kingdom
The Prime Minister is normally but not necessarily the leader of the political party which wins most seats after a general election - spaces on the chairs in the House of Commons. Voters vote for their own local MP, not for the Prime Minister.
The first Prime Minister was Robert Walpole in the eighteenth century, but he was known as the First Lord of the Treasury. The first person to be officially called "Prime Minister" was Sir Henry Campbell-Bannerman in 1905.