What is the full title of the Prime Minister of the UK?

The story behind the answer

The UK Prime Minister’s official office title is First Lord of the Treasury. Historically, the head of government emerged from leading the Treasury, and the title stuck even as “Prime Minister” became the common name. That’s why the brass plaque at 10 Downing Street reads “First Lord of the Treasury,” and the PM is also typically Minister for the Civil Service.

A quick memory hook: think “Number 10 = Treasury chief.” The other options aren’t head-of-government roles—Duke of Cambridge is a royal title, Her Majesty’s Loyal Opposition refers to the largest non-government party, and the Crown Estate is managed by an independent board, not the PM.