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

The full title of the Prime Minister of the United Kingdom is "The Right Honourable, First Lord of the Treasury." The reason for this title is rooted in historical traditions. It reflects the fact that historically, the main responsibility of the Prime Minister was to manage and control the government's finances as the head of the Treasury. Over time, this role expanded to encompass broader leadership and decision-making powers within the government, but the title of First Lord of the Treasury has endured to symbolize the Prime Minister's historical fiscal authority.