I wonder if it is this: https://news.cityoflondon.gov.uk/epping-forest-silver-birch-grove-planted-in-memory-of-her-late-majesty-queen-elizabeth-ii/. The story possibly fits, amd relates to Wanstead Flats but there's no specific mention of the name of the grove.

If the trees are Silver Birch then I reckon together with the plaque it justifies adding the name on OSM with the above as a website link. But it's sad that the City of London hasn't seen fit to mention the name and ensure it is searchable.