Tom: Splendid, I thought. What did you think?
Bernard: I, thought, splendid! What did you think?
Tom: Splendid, I thought.
Charles: Yes, it's odd, isn't it? All these years we've been single and proud of it and never noticed that two of us were, in effect, married all this time.
Tom: Traitors in our midst.
George the boor at The Boatman: I was at school with his brother Bufty. Tremendous bloke. He was head of my house. Buggered me senseless. Still, it taught me about life.





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 ★