I found this list of fireworks for the weekend.

http://www.viewlondon.co.uk/london-firew...plays.aspx

I'm torn between Crystal Palace on Friday (only a fiver) or Blackheath on Saturday (free) or maybe both. No sign of fireworks in Brockwell Park, but Southwark Borough are having a free display in Southwark Park on Friday (walking distance from Canada Water tube).

As for Lewes, it's a local festival for local people and outsiders are discouraged from attending as I was told by a friend who goes every year with family ties in Lewes.