Wikipedia
The First Lord of the Treasury is the head of the commission exercising the ancient office of Lord High Treasurer in the United Kingdom, and is now always also the Prime Minister. This office is not equivalent to the usual position of the " Treasurer" in other governments; the closer equivalent of a Treasurer in the United Kingdom is the Chancellor of the Exchequer, who is the Second Lord of the Treasury.
Usage examples of "first lord of the treasury".
It was not everyone who had a First Lord of the Treasury, Lord Chief Justice, Commander in Chief, Great Scientist, Master of the Buckhounds, Groom of the Backstairs and the Lord Mayor to accompany them.
As for the king, he made up his mind, with what courage he could summon, to meet the demands of the case, but wondered whether he could with any propriety require the First Lord of the Treasury to take a share in the burden laid upon him.