Well, I'm not sure that Sera took British habits from James but I agree with you on the fact that he took care of her. Two elements, the hotel where Sera live is paying by a friend more or less rich but not quite as Sonoko's family. James job's on the FBI make that he can afford it. Plus, where Conan didn't saw that Sera was a girl the first time he met her, despite his observations skills, James Black didn't hesitate about her gender. This could explain that Sera's dad was a great friend of Black on the FBI and died on duty but I think there is something more about him. This could imply that maybe James knows more than he's saying about the Mysterious Girl and the Organization. But its to soon to theorize about it yet.