It's all about English Gang Mentality.
On the London commute I don't acknowledge other cyclists (too many of them) unless something happens to bring them into the gang, such as a motorcyclist mouthing off (you can exchange a roll of the eyes), or if someone has a clipless moment in front of you (happened to me the other day and I went "oooooo are you alright" really loudly, thus making me just as embarrassed as him!)
It's a different story out on the open road - when touring I always acknowledge other cyclists with a smile and a nod of the head, cos they are in your "gang". Same with cycling on country roads.