You use the words “assuming Sr was his father”; according to the divorce papers there was a marriage automatically making Sr the legal father no matter who the actual sperm donor was, so the point is correct that his father was a British citizen!
That's right. Here it is:
Barry's Parents Divorce
The 1964 divorce record from the Circuit Court of the First Judicial Circuit State of Hawaii, states that Stanley Ann D. Obama and Barack H. Obama "were lawfully married in Wailuku, Maui, State of Hawaii, on February 2, 1961, by a person duly authorized to perform marriage cerimonies and ever since that date have been and are now husband and wife."
http://www.scribd.com/doc/18130289/Obama-1964-Divorce-Papers-13-Pages-Missing-Pg-11
However, if discovery was ever had in court, it would almost certainly be determined that his parents marriage was a polygamous or bigamous one as Sr. was previously married (and remained so at that time) to his first wife in Kenya. Therefore, his parents marriage could easily be found null and void. Barry would then, not have inherited British citizenship from Sr.
Of course, he would then have to confront the issue of the altered government document (the fake COLB) posted as truth, on his very own campaign web site. Lots of legal issues there, some of which I would presume are felonies.