Continuity mistake: When Scarlett and Charles miss their exit on the way to the first wedding, the highway where we see her reversing is not the same as where she came to a screeching halt a moment earlier. (00:03:40)
Continuity mistake: Half way through Charles and Hen's conversation after Charles comes out of Bernard and Lydia's hotel room a small dark spot appears on the shoulder of Hen's jacket, presumably spit from a previous take of Charles being hugged by her, but before it happens in this take. (00:53:20)
Continuity mistake: When Charles is looking for a wedding present for Carrie the sales lady hands him a list. He puts it back on the desk and walks over to a pygmy warrior, but the list is back in his hands when the camera shows them again. (00:57:15)
Continuity mistake: Carrie and Charles meet his brother at a cinema. When Carrie leaves and they walk into the shop Charles' loose shirt gets half tucked into his shorts from one shot to the next. (01:03:50)
Continuity mistake: After Carrie leaves Charles at the cinema he decides to run after her. When he catches up with her they stop on a wide sidewalk, and when he mentions how well he had prepared what he had just said to her, lots of people are walking by when the camera is on Charles, but the street is almost deserted when the camera cuts to Carrie. (01:05:15)
Continuity mistake: After Simon Callow's funeral, when Hugh Grant and James Fleet are talking by the sea, Hugh is holding up an umbrella. In the shots where the camera is in front of him, the umbrella is resting against the side of his head. However, when the camera changes shot and goes behind him, he is holding the umbrella up nearer the front of his forehead. (01:27:10)
Continuity mistake: When Charles finally walks through the vestry door to 'face the enemy' his hair changes from rather messy to nicely combed, then to messy again when he walks through the church. (01:40:55)
Continuity mistake: At the end of the film, after Charles has said "no" to Henrietta in church, Carrie comes to see him. It's pouring rain. He opens the door, being completely dry, decides to go outside to talk to her, and in the next shot about two seconds later, he's absolutely soaking wet. (01:46:40)
Continuity mistake: As Scarlett and Charles are heading to the first wedding, they pass a blue Ford on the motorway, just before they miss the turnoff. They reverse and head off the motorway, as the Ford continues on. When they get to the Church, the same blue Ford is parked on the left as they stop.
Continuity mistake: At Charles' wedding, when he's thinking about not going ahead with the wedding, the top button of his waistcoat is visible, but, seconds later, it's gone.
Answer: A groom can choose whoever he wants as best man. As you say yourself, it's by no means mandatory for it to be the groom's brother, even if they are close. In this particular case, it's possible that David felt that he ought not to be the best man, due to the difficulty that he'd have in making a speech, but, really, Charles was under no obligation to choose him.
Tailkinker ★