Yet I wonder why at the beginning A D shifted focus from Mr and Mrs Bennet talking to the men arriving-and on horseback. Dramatic effect?

According to the extra piece on my DVD and the book Making of Pride and Prejudice, by Sue Birtwistle and Susie Conklin, BBC/Penguin books. Andrew Davies said his goal was to remain true to the tone and spirit of JA's writing but good visual storytelling is at the heart of a memorable film. He therefore wanted to use the camera to tell parts of the story in a different and quicker way than a narrator would.

The shot at the beginning was written by AD in his words..
"..I wrote in a little opening sequence, which isn't in the book at all. It shows Bingley and Darcy riding their horses, and Bingley deciding to take Netherfield. It then moves to Elizabeth seeing them from perhaps half a mile away, and thinking.'Oh, there are two chaps on horses!' Of course she doesn't know who they ar but, almost as if inspired by the galloping of their horses, she turns and runs downhill towards Longbourn. SO right at the beginning one's trying to espress some of the this vitality."