By convention, the Prime Minister serves as the "First Lord of the Treasury," and the Chancellor of the Exchequer serves as the "Second Lord of the Treasury." Other members of the Government (usually whips in the House of Commons) are appointed to serve as the junior Lords Commissioners.
The English Treasury seems to have come into existence around 1126, during the reign of Henry I, as the financial responsibilities were separated from the rest of the job that evolved into Lord Great Chamberlain.
Today, the First Lord of the Treasury is as a rule the Prime Minister, and the Second Lord of the Treasury is the Chancellor of the Exchequer, who has inherited most of the functional financial responsibilities.