For a simple explanation of why Barack Obama isn’t a Natural Born citizen, watch the following video.
http://www.youtube.com/watch?v=GqH7rSHcvgU
The following is a link to British Nationality Law 1948.
http://www.uniset.ca/naty/BNA1948.htm
It clearly states that a person born after the commencement of the act would be a citizen of the UK and Colonies by DESCENT if the father was a citizen of the UK at the time of birth.
Please refer to Part II - Citizenship of the United Kingdom and Colonies - Citizenship by birth or descent.
5.—(1) Subject to the provisions of this section, a person born after the commencement of this Act shall be a citizen of the United Kingdom and Colonies by descent if his father is a citizen of the United Kingdom and Colonies at the time of the birth:”