I have read about the London Thames Festival in September. It is my understanding that the last Sunday of September involves boats and music on River Thames and wonderful fireworks display. I am assuming there are large crowds of people for this event and want to avoid mass confusion but enjoy the festivities in a less congested area.
I work in London and I don't recall hearing anything about things festival last year, but goggling it, it appears to be a month long festival throughout September. The events for 2015 have not yet been announced. The events take place over the 42 miles that the Thames runs through London, so you may have to travel quite a way to get to some of the events.
The only fireworks that I can see from last year were fireworks at the end of a classical music concert out at Hampton Court. There is a big fireworks display on the Thames on New Years Eve.
This now has the just as odd name 'Totally Thames'. The programme is to be released tomorrow (13 July).