The "Jack" part of the name may have come from the name of King James, who acceded to the thrones of both England -as James I- and Scotland -as James VI-, upon the death of Elizabeth I of England, who had died unmarried and without descent. From 1801, in order to symbolise the union of the Kingdom of Great Britain with the Kingdom of Ireland, a new design, which included the St Patrick's Cross, was adopted for the flag.
A white border was added around the red cross for reasons of heraldry. (The rules of heraldry demand that two colours must never touch each other.) The cross of St. Patrick was inserted so the position given to St. Andrew's Cross in one quarter was the same as that given to the Irish one in the diagonally opposite quarter; in heraldry this is known as "counterchanging". The British flag is not symmetrical because of the counterchange.
The Welsh dragon does not appear on the flag because when the first Union Flag was created in 1606, Wales was already united with England as a Principality.
