It's a wartime air recycling box. During the Second World War the Germans used U-Boats to run large-bore pipelines across The Channel. At Calais they installed massive air pumps using the slave labour of Vichy French convicts and Eastern Front POWs. The plan was to suck our English oxygen over to their side and reduce our respiratory morale.
Churchill's reposte foiled all of that, though. With the help of a Home Office man called Partridge Whittington-Costco, the PM invented, and installed by hand, these Oxygenglish Airy Repurifiers at many places around the south coast. The idea was that the local residents would exhale into the box's slot whenever they felt giddy or depressed. The OARs worked, very well indeed, and the Nazis were defeated as a direct consequence. Although they fell out of use once the War was over, the boxes' green colour lived on as the hue most associated with recycling.