Who do i complain to about a chinese establishment selling multi pack cans of pop for 2 pounds a go.?

My wife and i went to our local Chinese to order take away. They have a bar so we decided to have some soft drinks while we waited for our meal. I had a pint of draught coke while my wife had a can of diet Irn Bru. The waitress poured some in a gl then gave my wife the can which on the side said from multi pack not for resale. Is this against the law? Can i complain to anyone?