Trivia: If you think the church where Charles nearly marries Henrietta looks familiar, that's because it's the same church where the Sheriff of Nottingham attempted to force Maid Marion to marry her in Robin Hood;Prince of Thieves-St Bartholomew the Great in London.
Trivia: In the scene where Charles has run into Henrietta in the hallway after witnessing Bernard and Lydia having their sneaky consummation, Henrietta pulls Charles' head onto her shoulder. Look closely and you can see Hugh Grant's lip get caught quite badly on the fabric.
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 ★