Episode #1.5 - S1-E5
Trivia: Tommy Flanagan plays Arthur Shelby Sr, father to Arthur (Paul Anderson) and Tommy (Cillian Murphy). In real life, Flanagan is only 11 years older than Murphy and 13 years older than Anderson. (Incidentally, Arthur is 3 years older than Tommy despite Anderson being 2 years younger than Murphy).
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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