You're probably going to have to go from London. To be honest, I'm from Scotland and I didn't even know that Aberdeen had an airport. I doubt it does flights to Tokyo. You can use a website like these ones to compare flight prices and stuff:
September is the end of holidays. I don't really get it unless it's lots of students going back to the UK/Japan and business types going back to work. I would have thought that June - August would have been more expensive.