View source for Act of Union (1707)

Even though the English and the Scottish crown were united under [[James I]] (Scottish James VI), 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

http://www.parliament.uk/actofunion/01_background.html