I think the best view is from Blythe Hill Fields as it is largely open and points out towards Lewisham and Blackheath. We went up to Blythe hill a couple of years ago and it was a really good atmosphere with lots of families and kids out looking for the fireworks, and some bringing their own to the party!
The best thing about seeing the fireworks from Blythe Hill is the quick walk down to the Blythe Hill Tavern, a king of pubs :-)

Isn't the main view from One Tree Hill out towards the city and not east enough to see Blackheath clearly?