Often at BA you don’t stay with the Cabin Crew and don’t actually perform a formal cabin crew brief so it is feasible the first time the pilot encountered this crew member was during a toilet break or her visiting the flight deck. I had this once myself when a crew member came into the flight deck and smelled like a brewery. I simply said to her that I suggest she reports to the CM that she is too unfit to operate and sit herself in the back row for the remainder of the flight or the other option would be a breath test on arrival. She removed herself, I had a further discussion with her after landing saying that if she did it again it would be the police. I can’t see much point in ruining someone’s life without giving them a chance. I also have had a mate that suggested the same to his FO at report I.e you should go unfit for duty or I will require a breath test before we proceed.