Plot hole: When we see the murder happen in flashback, the killer stabs the victim wearing no gloves whatsoever, nor wipes the handle. In a very sportsman conduct, the victim also decides not to literally scream bloody murder as he gets stabbed, nor make any sort of sudden, noise inducing movement that would have instantly exposed what was happening. He gives their killer, apparently, all the time to go back to their accomplice before Catherine notices the blood pouring on the floor - how rude of him to silently bleed all over the booth without cleaning after himself. (00:41:45)
Plot hole: The murder happens where and when it happens because the candidate "is a very busy man", and apparently then the best course of action to kill him is doing it while he is casting his vote. At this operation, involving the other 2 candidates for the role, there is no press nor any normal voter, for no reason - not safety since Dwayne was not expected. Had they introduced the rich Victor Pearce as some sort of mobster surrounded by bodyguards, it would have been an acceptable plot idea, but the guy travels with his son as sole member of the staff and nothing about his characterization leads the viewer to believe that the only chance to murder him is for a rotund 62 years old lady in clogs to perform a Metal Gear stunt sneaking in undetected while a priest is facing the other way for 5 seconds and pray that nobody else shows up at the voting booth and all the others are taking their time to put a cross on a piece of paper.
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.