Page "Politics of the United Kingdom" Paragraph 29

The monarch appoints a Prime Minister as the head of Her Majesty's Government in the United Kingdom, guided by the strict convention that the Prime Minister should be the member of the House of Commons most likely to be able to form a Government with the support of that House.

In practice, this means that the leader of the political party with an absolute majority of seats in the House of Commons is chosen to be the Prime Minister.

If no party has an absolute majority, the leader of the largest party is given the first opportunity to form a coalition.

The Prime Minister then selects the other Ministers which make up the Government and act as political heads of the various Government Departments.

About twenty of the most senior government ministers make up the Cabinet and approximately 100 ministers in total comprise the government.

In accordance with constitutional convention, all ministers within the government are either Members of Parliament or peers in the House of Lords.