Character mistake: A computer display of currency transactions shows the word "withdrawal" spelled as "withdrawl." (It is also spelled correctly higher up on the display). (00:25:05)
Plot hole: The murder as described makes kinda sense (it's a device used in other works of fiction), but not at all as shown; the person hiding in the bathroom closes the door to the point of making it click, and opens it also with a click. Even panicked, the other person in the room is going to hear the sound of a lock right behind him. The door should have been closed only partially. Moreover, the bathroom door is right by the front door closed just by the door chain, and Philip Marston had the door unlocked and with just the chain to break; the bathroom door was fully in his view the whole time.
Other mistake: The text message used as ploy to cover up the murder was sent at 17:44, but during the flashback showing the killer setting up the murder scene, a wristwatch on the vanity is at about 10 o'clock. (00:44:25)
Continuity mistake: After the obligatory ultimatum visit by the Commissioner, Jack asks Florence for good news. When she chirps that she -really - found something, her hands drop in the close-up, but are holding the headphones steady and up in the next shot. (00:32:25)
Character mistake: Florence grabs the victim's phone with a tissue to avoid leaving prints...but then uses her other bare hand to go through her messages via touchscreen. In many other scenes she's just seen wearing latex gloves. (00:12:30)
Continuity mistake: Dwayne's bushy cover has been blown, so he nonchalantly walks out and greets the stern-looking Audrey. He opens his arms when he says her name, but in the reverse shot he repeats the movement, as he was still pretending to scribble on his pad. (00:14:00)
Continuity mistake: After the chat with the hotel manager, Jack and Florence are in Stephen's room. The level of drink in Stephen Marston's cocktail glass changes between shots - it's half full when Jack is straining to sit down, and is back up right at the cut. (00:35:25)
Continuity mistake: DI Jack Mooney is looking at the coroner report on the PC screen. He changes his stance and is initially hunched over and then straight, but the person in the foreground is keeping the exact same posture every time the camera cuts to Dwayne. (00:25:35)
Character mistake: The forensics report in its first screen displays the word "antybodies", when of course it is "antibodies" (00:25:15)
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.