If you walk along the Thames Path, east of Woolwich, you will discover these small red lighthouses. They can be found at Margaretness (or Tripcock Ness), Crossness and Coldharbour.
They are not lighthouses, in the traditional sense, but more like navigational light masts.
The Margaretness Lighthouse was installed in 1902, by Trinity House (the General Lighthouse Authority for England), but has been operated by the Port of London Authority, since 1993.
In 1566, Queen Elizabeth I empowered Trinity House to set up "so many beacons, marks and signs for the sea whereby the dangers may be avoided and escaped and ships the better come into their ports without peril."
