Episode #3.1 - S3-E1
Continuity mistake: At the wedding, Arthur leaves the table looking for Tommy and Grace, leaving his suit jacket. The next scene, he appears at Tommy's door wearing the jacket. (00:14:20 - 00:15:30)
Episode #3.2 - S3-E2
Continuity mistake: John and Polly are talking in his office. He is wearing a ring on his left hand, then a few seconds later he is not.
Other mistake: There are multiple spelling mistakes in the sub-titles: Saltley is spelled Saltely, Balsall Heath is spelled Basall Heath, and Changretta in Italian would be Ciangretta.
Factual error: Mosley is portrayed as forming the British Union of Fascists in 1929 or 1930. In fact, he formed the New Party in 1931. The party failed to win any seats in the 1931 general election (with Mosley himself losing his seat) and Mosley then formed the British Union of Fascists in 1932.
Answer: There has never been prohibition in England. There are many reasons for running an underground distillery. It would be a way to avoid things like government regulations, safe distilling methods, alcohol content limits, taxation, fix pricing, and so on.
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