Aircraft recognition was a rather popular pastime in civilian Britain during WWII, while in the military, it was seen as a necessity. In the 1930s, the main nucleus of aircraft recognition was the Observer Corps. This was a civil defence organisation, founded in the 1920s to spot and track aircraft flying over the UK in case of war. Its members were well trained in aircraft recognition.Beyond this, aircraft recognition was mainly the province of enthusiasts. However, with the beginning of the war, aircraft recognition became a lot more common. In 1941, Penguin would publish a paperback recognition guide for the civilian market. It was written by R. A. Saville-Sneath, a major figure in the Observer Corps, and it would sell over seven million copies. The Ministry of Information produced pamphlets describing British aircraft, which were also popular - in the prologue to his Six Armies in Normandy, John Keegan reminisces about obsessing over them as a young boy during the war. There were other forms of recognition guide - you've mentioned posters, but the Air Ministry also produced recognition playing cards, while on the private side, cigarette cards with depictions of aircraft were common.

This information came from a variety of sources. A lot of knowledge came from pre-war sources - airshows and the like. Much as they do today, air forces took the chance to show off their flying abilities and the capabilities of their aircraft. However, this allowed other militaries, and civilian sources, to get good looks at new aircraft. During the war, espionage took somewhat of a back seat. Images of new enemy aircraft could come from several sources - propaganda photos of new aircraft were surprisingly common. Descriptions given by, and photos taken by, pilots encountering new aircraft were highly useful. Finally, as you note, crashed and downed aircraft gave a way to identify new types.