The circle of traffic tightens at this hour round Regent’s Park; cars hummed past without a break; it was just before lighting-up time – quite soon the All Out whistles would sound. At the far side of the road, dusk set the Regency buildings back at a false distance: against the sky they were colourless silhouettes, insipidly ornate, brittle, and cold. The blackness of windows not yet lit or curtained made the houses look hollow inside. (pg. 9)
In this part, what does "set the Regency buildings back at a false distance" mean? Particularly I'm not sure about "false distance." Also I don't understand "the All Out whistles." I looked up the dictionary and googled this expression but to no avail. Can somebody explain them? By the way, the setting of this novel is 1930s London. I appreciate your help.