It's not entirely clear if you realize this, so worth mentioning that BA does not fly non-stop from Chicago to Japan. You'll find code-share flights that have BA flight numbers, but you'll be on an American Airlines plane. Otherwise, you're looking at a very long trip from ORD-LHR-NRT/NHD. Figure you have an 8-hour flight to London and a 12-hour flight from London to Tokyo. In contrast, non-stop from Chicago to Tokyo is about 13 hours.