Traditionally there has always been a really good one in Battersea Park. Though last year Wandsworth charged for entry - so we did not go. (Don't drive though as parking is a nightmare, better to take the train from Waterloo to Queenstown Rd or walk from Vauxhall.)
Lambeth normally have one in Kennington Park but cancelled it last year because of 'security'. There was a bit of an outcry, particularly as it was hard to find the basis for this perceived security problem. My guess is that it will be back this year.
From the Chair of Friends of Brockwell Park, and the Chair of Lambeth's Environment Forum, no less, at the last meeting of Lambeth's Parks Forum. (A pretty doomed body, but that is another story.) Lambeth have apologised and promises that it won't happen again, but Kennington fireworks appear doomed. Apparantly there is not a single site in the north of the Borough that is suitable, because space is needed for the fall out from rockets etc. I suggested somehting over the river - other boroughs seem to manage, but.... (so maybe it is the money as well).
Battersea this year costs £3 and they have promised a light show as well as the traditional. And kids under 10 are free.