Character mistake: Several times Tommy addresses Sir Oswald Mosley as Mr Mosley instead of the correct Sir Oswald. Mosley never corrects him, which given his snobbishness portrayed throughout the series seems highly unlikely.
Character mistake: Mosley says Tommy won the Distinguished Service Medal. This was a naval decoration. Tommy actually won the Distinguished Conduct Medal. Not a mistake someone like Mosley is likely to make.
Character mistake: Ada is listed on the portraits of The Company officers as Mrs Ada Shelby. She is actually Mrs Ada Thorne.
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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