I just watched The Dig on Netflix, which is the dramatisation of the Sutton Hoo excavation in Suffolk immediately prior to the outbreak of war. There are several scenes which emphasise the presence of WW2 on the home front, such as a child running round playing while wearing a gas mask, scenes in London where windows and statues are being sandbagged, and a scene in a pub where the windows are already being blacked out as a rehearsal. All of this is happening before Chamberlain’s famous radio speech explaining that war had been declared.
So is this accurate? My understanding was that although it was common knowledge that war was a possibility, and measures such as gas masks, the blackout and sandbagging to protect against bombing are well known, but I was surprised to see these measures being portrayed so early on.
British preparations were relatively well advanced by 1939; the fear of aerial attack was a persistent thread of the inter-war years, typified by Stanley Baldwin's 1932 "The bomber will always get through" speech, and poison gas was expected to be a key component of aerial bombardment. As international tensions escalated the government instructed local authorities to draw up Air Raid Precaution (ARP) schemes, written into law as the 1937 ARP Act that came into force at the start of 1938. Despite that some councils had not started to put plans together by the time of the Munich Crisis of September 1938 when war seemed imminent.
Though Chamberlain returned from Munich declaring "peace for our time", preparations for war accelerated. John Anderson was appointed Lord Privy Seal with particular responsibility for ARP in October 1938 and the number of volunteer ARP workers had risen from 200,000 in June 1938 to almost 1.6 million by July 1939. Pamphlets were issued to all houses in July 1939 outlining air raid warnings, lighting regulations, use of gas masks, evacuation, rationing and such; the scenes from the film are all plausible. There's another where coloured balls are being thrown from a car to represent different types of bombs, not a widespread practise as far as I'm aware, but not entirely out of keeping with pre-war exercises, e.g. from Richard Overy's The Bombing War: "In the summer of 1939 the Hull ARP organization ran regular training demonstrations of bombs and their effects in local parks; after detonating a miniature but very noisy high-explosive bomb, the training focused on gas bombs. Persistent gases (mustard gas and lewisite) were identified by smell; non-persistent gases were released in small quantities and the trainees warned to stand upwind while they observed its passage."