A few of these on an oak leaf at the edge of the grassland SE of Alexandra Lake.

Difficult to identify from a photograph, but the thorax has a reddish hue whereas the head does not. The most likely candidate is Formica cunicularia, which has been recorded from Wanstead Flats (Essex Field Club), and the habitat would likely suit it.

I have also found what appear to be the same species in the City of London Cemetery and - interestingly - on Manor Park Station. (see here)

At least until work began in 2017 in preparation for the new Elizabeth Line, the colony at Manor Park Station was located in rubble-based soil at the bottom of the embankment adjacent to Platform 4. Whether these will survive the extensive work remains to be seen.