Admittedly not a county but the city does have it's own historical flag.

The City of London Corporation is not to be confused with Greater London which surrounds it. The city administers itself, has it's own Police force and is only 1 square mile. It is often confused with the City of Westminster in which the Houses of Parliament stands.

If ever you are in that part of London and you see the bollards have painted red and white tops with a crest, that is a sure sign you are in the city!

The flag is a cross of St George with a dagger in the top left hand corner, this symbolises the dagger Sir William Walsworth used to kill Watt Tyler with during the 1381 peasants revolt.

Although Stuart Johnson has written in to say:

I noticed that on the page of the City of London's flag, you state that the flag is a St George's cross with a dagger in the top left hand corner, which symbolises the dagger Sir William Walsworth used to kill Watt Tyler with during the 1381 peasants revolt.

This is not the case - it is a sword, not a dagger that appears in the canton.  And the sword represents  the sword that beheaded Saint Paul who is the patron saint of the city.