Not an ant at all, but a type of solitary wasp. This Velvet Ant was photographed in Wanstead Park in 2008, but I didn't know what it was until I recognised it from a photograph in 2014.

It is an attractive creature to look at; the males are fully winged but the females - as here - being wingless. It doesn't build nests, but lays its eggs in bee nests where the larva feed on the pupa of the bees.