Saw this photo in a gallery of Photographer Jason Hawkes work.
These are the Maunsell Sea Forts, they were small fortified towers built in the Thames and Mersey estuaries during the Second World War to help defend the United Kingdom. They take their name from their designer Guy Maunsell. After they were decommissioned in the late 1950s, they were used for other activities, and one became Sealand, the self-described “micronation”.