Wednesday 12 August, between 6:17pm and 8:06pm (peak at 7:12pm).
Make sure you have appropriate eye protection and a good westerly view (such as Horniman Park or Westwood Hill). Horniman Park closes around 8:30pm in the evening, so there is still time to watch it before the park closes.
More details on how to view it at Solar eclipse: The best places to see the event in London - BBC News