He uses his British passport. Because he is British. Why would he use an American passport when he is a British citizen? He is not culturally nor emotionally American, nor is he bound in honour to America. He is a British man, and in fact is a member of Parliament. He has a tick against his name because of where he was born and this tick is stopping him from being British - in the eyes of American passport authorities. This is the point of his essay.
You tell me. Why did he use it in the past?