Look closely at the door of 10 Downing Street
You will see in the image above that engraved on the letterbox of 10 Downing Street is “First Lord Of The Treasury”.
At the beginning of the 17th century, the running of the Treasury was frequently entrusted to a commission, rather than to a single individual. Eventually in the middle of the same century, the First Lord of the Treasury came to be seen as the natural head of the overall ministry running the country, and, as of the time of Robert Walpole (Whig), began to be known, unofficially, as the Prime Minister. The term Prime Minister was initially, but decreasingly, used as a term of derogation: it was first used officially in a royal warrant only in 1905.