Act of Union (1707)
The Act of Union
Even though the English and the Scottish crown were united under James I (Scottish James VI), but there was no political union. There were two parliaments, legal systems, education systems, etc. England and Scotland had separate governments. The Act of Union was installed as a result of the Act of Settlement on the 1st of May 1707 in order to prevent a separation of the two countries (and an attack by the Jacobites). According to the Act of Settlement the Hanoverian house should succeed Queen Anne I. The Hanoverians could claim the English throne, but not automatically the Scottish one. Therefore, England and Scotland were united under the Act of Union and formed the United Kingdom.
Sources:
http://www.royal.gov.uk/HistoryoftheMonarchy/Scottish%20Monarchs(400ad-1603)/TheStewarts/Anne.aspx