The prime minister runs the British government, and the job exists almost entirely by convention. There is no statute that creates it, no separate election for it, and no fixed term attached to it.
How you get the job
You become prime minister by being the person who can command a majority in the House of Commons — normally the leader of the largest party. The monarch formally appoints you, but has no real choice in the matter.
This has a consequence visitors find startling: the British do not vote for their prime minister. They vote for a local MP. If a governing party changes its leader, the new leader becomes prime minister immediately without any public vote. In 2022 Britain had three prime ministers — Johnson, Truss and Sunak — and no general election.
The powers
Considerable, and mostly informal. The PM appoints and dismisses ministers, chairs Cabinet, sets the agenda, controls the machinery of government, and exercises most of the remaining royal prerogative powers in the monarch’s name.
What checks them is the party. A prime minister who loses the confidence of their own MPs is finished, however recently they won an election — Thatcher and Johnson were both removed by their own side. British politics has no impeachment because it does not need one; the mechanism is faster and less formal.
First among equals
The traditional description is primus inter pares — first among equals in Cabinet. In practice the office has grown steadily more presidential since the 1980s, with power concentrating in Number 10 and its advisers, and complaints about “sofa government” have been made about most recent occupants.
Where the title came from
Robert Walpole, who dominated government from 1721 to 1742, is counted as the first prime minister — though the title was originally an insult, implying he had raised himself above his fellow ministers. It was not used officially until 1905.
The residence
10 Downing Street is both home and office. The flat above the shop is famously small, and prime ministers with families often swap into the larger flat at Number 11, which is nominally the Chancellor’s — an arrangement that has caused more than one newspaper story.
The weekly audience
Every week the prime minister meets the monarch privately. Nothing is minuted and no officials attend. It is the one conversation a PM can have entirely off the record with someone who has no political stake and decades of institutional memory, and prime ministers of both parties have described it as unexpectedly valuable.