Wordorigins.org

John Bull is the English equivalent of Uncle Sam, a personification of England and the English people.

The term dates to 1712 and first appears as a character in John Arbuthnot’s satire The Law Is a Bottomless Pit. By the end of the 18th century, John Bull was being used as a metaphor for England. From a 1778 letter from John Adams to his wife, Abigail:

France...assisted the American cause, for which John Bull abused and fought her. But John will come off wretchedly.

The familiar appearance as a top-hatted, paunchy, Victorian gentleman dates to the nineteenth century and the cartoons by John Tenniel (perhaps most famous for his illustrations for Lewis Carroll’s Alice in Wonderland).

(Source: Oxford English Dictionary, 2nd Edition)