No, it is very UNLIKELY that he was a British Citizen if born in the US as Obama Sr was already married and could not legally marry Ann.
Heritage is not determined by Marital Status.
That may or may not matter. It's not that he, BHO Jr, was a British Citizen at birth, but rather that his father was not a US citizen. If BHO Sr was listed on the original BC as the father, even if they weren't married, he's legally the father. Of course because he was, under this assumption, born in the US, little BHO was a citizen at birth, but likely not natural born. Same if he was born outside the US, he'd be a citizen, if his parents weren't legally married, but not natural born, because it's a statue that makes him a citizen in that case. If born outside the US, and parents married, the statue makes him not a citizen at all, because Stanley Ann failed the US residency test, which as you know, is different if the mother is married than if she is not.