Shaggy Soldier (Galinsoga quadriradiata). This South Ameriacan species, now naturalised in places, is seen here growing in a crack between the wall of the launderette and the pavement in the parade of shops on Erith Rd., Northumberland Heath, Borough of Bexley.
Other locations I've found it in in London so far are Lewisham railway station, bits of Bloomsbury (including near ULU) and in raised beds at Euston railway station.
Its more lanky and less hairy relative, Gallant Soldier, is found on a Barnehurst, Bexley, allotment site.
Galinsoga are members of the daisy family.