It's here: https://commons.wikimedia.org/wiki/File:Wanstead%20Flats%2C%20Coronation%20Grove%2C%20plaque%2001.jpg?uselang=en-GB

Part of it's broken off, but I make it: "These trees ??? emorate the coronation of Her Majesty Queen Elizabeth II 1953 ??? raised by ??? ubscription."

Plausibly: These trees commemorate the coronation of Her Majesty Queen Elizabeth II 1953 funds raised by public subscription.

Googling "Coronation Grove" and "Wanstead Flats" implies that at least some people are using that name for something somewhere on the flats.