I think you are probably right. Although this is a matter of a bit of obscurity due to the changing laws on the subject, the British law that made Obama a British colonial subject at birth may not have operated to that effect if he had been born out of wedlock to his British father.
I think the evidence is that Obama's parents WERE married (otherwise a divorce would not have been needed) and that Obama WAS a British subject at birth, and that he was not, therefore, a natural born citizen of the United States. I believe that this is the classic example is who the Framers intended to bar from the Presidency by their use of the phrase: someone who could be claimed as a national by Great Britain.
Whether Barry himself was a British national at birth under British law, is of no import. The citizenship status of his father at the time of his birth is what counts, at least under the Vattel "Law of Nations" definition of "Natural Born Citizen".
The natives, or natural-born citizens, are those born in the country, of parents who are citizens.
BHO Sr was not a US Citizen, so BHO Jr cannot be a Natural Born US Citizen. (That quote is from the 1852 translation by Chitty, however. The original French said "The natives, or natural-born citizens, are those born in the country, of parents who are citizens." The original French said "naturels, ou indigènes" apparently the translator turned the order of terms around. The earlier, British, translation said "natives, or indigenes". The founders would have read the original French version anyway.