I'd go one of two ways;
1. Ask him to indicate to me where the law or the highway code compels me to give way; he won't be able to, so I laugh and cycle away, shaking my head in wry amusement (to tick him off even more)
2. Tell him if he wants to argue about, to pull over and get out of the van; he's unlikely to - if he doesn't, I laugh and cycle away, shaking my head in wry amusement (to tick him off even more). If he does, I knack him.
Bren