Personally I'd consider it a non-incident. Perhaps I'm desensitised and/or missed an earlier exchange/horn beep from the van chap. As much as the dream is for everybody to perfectly follow the highway code, with everyone on top of each other and London's competitive nature, the etiquette/legal rules tend to be bent a little, for all road users. Not suggesting it's right, but it happens.
I say (only my opinion) just stay calm, be patient and courteous. It's amazing the small things I see some people fly off the handle over. If commuting in London, every day you'll get 'numpties'-- often best to anticipate them and take the safest course of action. Ok the van driver in this instance is a little ignorant/uneducated in the way to share the road with cyclists, but they'll never improve or consider their driving if made more angry. It's likely they'll end up shouting at the next cyclist they encounter, perhaps with more severe consequences.
If they've reeeally done something worthy of a talking to- best to be calm and measured whilst you explain what they've done to compromise your safety and why you're speaking to them. It might be they get shouty and/or dismissive, believing you're completely in the wrong- still, they're more likely to listen to you if you don't rise to that level and become flustered. Suppress that adrenaline ;-)
Neither individual (IMO) came out of it with any credit and both left the scene with preconceptions probably even more firmly entrenched.
