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 my garden - with numbers making use of cracks in the mortar of the bricks of my house - in the City of London Cemetery and - interestingly - on Manor Park Station. (see here)