The cross of St George, not the Union Jack, is the flag of England. It is a red cross on a white field.
The official proportions for the national flag of England is 3:5, with the cross being 1/5 of the height of the flag wide.
When King James VI of Scotland ascended to the English throne, thereby becoming James I of England, the national flags of England and Scotland on land continued to be, respectively, the red St George's cross and the white St Andrew's cross.
At sea, however, from about 1606 a combination of the two flags was used which was the predecessor to the current Union Flag (often called the "Union Jack" although technically a "Jack" is a naval flag only).
The combination of St George's Cross and St. Andrew's Cross was incorporated into the early (pre-indepencence) American flag.
The current flag of the United Kingdom of Great Britain and Northern Ireland which incorporates the Cross of St. Patrick (the diagonal red cross) was adopted in 1801.