The B.Y.O.C. bar in London, England, doesn't provide any alcohol. Instead, it asks that patrons brink in their own booze. What the bar will do for you is provide a mixing service complete with mixers, a bartender and a trolley. The bartender comes around and uses your drink of choice to mix cocktails.
B.Y.O.C. was forced to work with this business model because the quaint cocktail bar simply doesn't have an alcohol license. As a result, they are not allowed to sell any alcohol to customers.
Since the bulk of the cocktail's value is provided by you, every cocktail mixed is free. Instead, patrons pay an upfront fee of 20 quid before entering to cover the mixers and service.