You couldn't be soooooo wrong.
I've spent 20 years driving very large Left Hand drive cranes around the UK, I've probably pushed my way into more traffic than the lot of you put together
.......but slowing down on a main road when the way ahead is clear is not the thing to do, again while driving the crane and a car in front slows unnecessarily , causing me to brake, to let a car out will effectively take me about 3 miles to make it back up, cause a huge tailback and a whoping blast from the airhorns
I would have been looking beyond the car in front and after seeing there was no reason to slow down I would not have been prepared to brake,more than likely accelerating, even at less than 30mph it makes a huge difference to large vehicles.
None of us stick to all the rules, I don't, but one I do stick to is MIRROR, signal, manoeuver, which makes you aware of what's going on around you, which in turn should help you make the decision on what to do. So in this context i'd say the rules were pretty important, if he'd have checked his mirror before braking he would have seen angry white van man bearing down on him, probably changing his decision to brake. Angry van man was probably looking over the op's car and accelerating as there was a gap to go and no reason to slow. Yes it would have been white van mans fault if he had rear ended him.
Just pointing out that people have different agendas while driving
I more often than not when driving my car let cars out while i'm in slow traffic, but I always make sure that it won't affect other drivers, I will accelerate slower to leave a larger gap as i'd have judged the road ahead, not simply brake