These ants were photographed in the City of London Cemetery. Although difficult to identify from a photograph, and there are similar species, 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).
I have also found what appear to be the same species on Wanstead Flats
(see here) and on Manor Park Station.
(see here)