In the opening scene Collins is holding a mug of bitter (an amber beer). Moments letter, when leaving the pub, he empties a mug of mild (a black beer). Possibly only an Englishman would notice that.
There are many occasions where lights are shown at night, contrary to the blackout regulations.
During the title sequence Spitfire night-fighters (a very unhappy and short-lived experiment in 1940) are seen. With invasion-stripes from 1944.
During the title sequence the tail-fin of the German bomb dropping over London is that of a "Tallboy" - a British bomb. and not available in 1940. it also seems as if the bombers are dropping bombs nose-first. The predominant bomber during the Blitz, the He111, dropped them tail-first initially.
In his room after leaving his mild, he takes a young lady to his room which is far too well lit ( there was a war on) and no blast tape on the windows. When he is called to the first victim, the room is lit by no less than THREE 100w standard lamps.
Shortly after Collins states that the woman found in the bombed-out building had been dead for 48 hours, Molly tells the rent-boy that she had been killed last night.