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.
Continuity mistake: Dwayne is staring at the boards after his superiors have gone to the church function. He says "JP..." and his younger coworker has his pen on the paper, but at the cut the pen hand is scratching his nose. (00:34:00)
Revealing mistake: The photo from the high school years of Edwina and the candidate is a photoshop Frankenstein where you can see the face of the actress pasted on someone else's head. (00:24:50)
Other mistake: Days in the series seem sometime to go on for unusually long amounts of time. In this episode, there's the exact opposite; the murder happens at 10 AM (which already seems awfully late for the opening of an election), the police obviously are called right away, Jack and Florence go to the station to discuss the case, and then all of a sudden it's a wrap when Dwayne and JP get back there with their findings. Unless they have searched the businessman's residence for over 6 hours while Jack and Florence have been doing next to nothing at the station, it makes no sense.
Continuity mistake: When Florence "Ehh!"s and "Shh-shh!"s Dwayne, she does it with one hand, which becomes two at the next shot. (00:13:00)
Continuity mistake: Jack receives the photos of both Reverend Matthew Dawson and his wife; during the sequence, the black marker switches hand from right to left, to right again, and is pointing sometimes up, sometimes down. (00:11:20)
Continuity mistake: Back at the station, Jack picks Peter Baxter's profile off Florence's pile. Action happens in two shots, and the marker in his hand does a 180°; black cap up/black cap down. The mistake happens again later, with Catherine's picture. (00:10:50)
Continuity mistake: Fire exit scene. Everyone turns around surprised by Jack's appearance in the opposite direction. Florence turns twice, though. (00:07:20)
Continuity mistake: DI Jack Mooney arrives on the scene of the crime. He is walking with his left hand on his chest in the first shot, but viewed from the side, his arm hangs low. (00:05:25)
Continuity mistake: Catherine is chatting with the not-so-bad candidate, who offers his support. She says "I'd like that" with a bit of her scarf that goes in front of her shoulder, unlike the previous shot. (00:01:45)
Continuity mistake: After Judith Dawson's first testimony, Jack thanks her; Florence is just pretending to write and stops, but in the next shot she is back at it, scribbling her pad with the pen. At the end of this new shot, she stops writing, but is writing again in the next, all while DI Jack Mooney talks in continuity. (00:08:55)
Continuity mistake: At the beginning of the episode, Edwina Bousquet scoffs at the apology and gets up. Close-up of Judith Dawson, and you can see for a second in the background the bike with Danny John-Jules entering the premises. But there is no sound effect for it and in the next cuts he's nowhere to be seen; he arrives only later. (00:00:25)
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.