In addition to nice weather and fewer crowds than at the height of the summer high season, there's another reason to visit London in September. And that's "Festival Season." VisitLondon has a list of all the festivals and events during the month, including film festivals, a river race, the Greenwich Riverfront Jazz Festival, and an open house weekend that offers the public free glimpses into 500 buildings of architectural interest, and many others.
Many of these festivals and events are free. And judging by recent sales, airfare and vacation package prices are coming back down after an expensive summer. So there it is: London, festivals and events, and fall prices. It's your information now, do with it what you will.