Episode #2.4 - S2-E4
Corrected entry: Tommy Shelby says that he is exporting whisky to Halifax, Nova Scotia, which is under prohibition, as is all of Canada and the United States. There was no prohibition in Canada. Not a mistake a businessman as astute as Shelby is likely to have made, especially as he's based his whole business plan on it.
Correction: Except Season 2 occurs in 1921-22, well after Prohibition ended in Canada.