Otto, I'm a fireworks fan too and understand your frustration on hearing a show in progress nearby that I'd like to have seen close-up. I don't know of any site that gives advance notice of fireworks displays in London but these two may give details of some shows:
1. The Port of London Authority website lists firework shows if they'll affect river traffic (such as at New Year):
2. Ian Visits is an amazingly useful weekly listing of upcoming events in London which may include major fireworks displays:
That's all I can suggest.