This was a British unrequired plan for the naval defeat, by forces controlled largely by Admiral the Hon. Sir Reginald Plunkett-Ernle-Erle-Drax’s Nore Command, of any German seaborne landing on the east or south coast of the UK (1 July/31 August 1940).
As an initial step, light cruisers of Admiral Sir Charles Forbes’s Home Fleet were redeployed from northern waters to the waters off the south of England: Manchester of the 18th Cruiser Squadron was based at Portsmouth, and Newcastle of the 18th Cruiser Squadron at Plymouth. At the end of July Jackal, Jaguar, Javelin, Jupiter, Kelvin and Kipling (5th Destroyer Flotilla) were transferred to the south from Scapa Flow in the northern Orkney islands group to Immingham on the Humber estuary for invasion defence.
If any indication of a landing was received, the warning 'Stand by Operation "Purge" [then the location]' was to be issued, and if a landing was in progress the signal 'Carry out Operation "Purge"' was to be issued. On receipt of the signal all vessels in the threatened area would proceed immediately to the attack. The locations associated with this signal were to the north of Flamborough Head (Area I), between Flamborough Head and Skegness (Area II), between Skegness and Brancaster (Area III), between Brancaster and Lowestoft (Area IV), between Lowestoft and Brightlingsea (Area V), between Foulness and Shoebury (Area VI), the Nore (Area VII), between Sheppey and Thanet (Area VIII) and Dover (Area IX).