If you wanna see heavy cavalry dicking on infantry look at the Anglo-Norman conquest of Wales and invasion of Ireland. (particularly the battle of dublin in which 100 knights took on 1000+ Irish/Danish forces and won)
Hey, do you have any links (like wiki, or another site) that provides the context and procedure of these battles? I tried to look up battle of Dublin, and wiki only shows the 1922 version. I'm very interested in reading about these!
(longbow is an old old old weapon, and not uniquely British by any stretch of the evidence, the only advantage the British ever had was numbers, and even then it's debatable if they worked against plate).
You are right that bows that were long are not unique to England. However, the "English longbow" was unique for more than just their numbers. First, the Yew trees that grow in wales produce a superior bow than hickory, ash, or other woods commonly used in bow making throughout continental Europe. There are other all-wood bow designs that are more efficient at transferring energy to the arrow (like a native american flat-bows) but those highland Yew trees cannot be found in other regions and other people would be hard pressed to find a substitute that could posess the same power -- relying instead on composite materials to acheive the same draw-poundage (however composite bows do not preform well in damp conditions, which is their disadvantage)
Another uniqueness is the archery culture of England. Banning other sports and requiring all common men to practice archery each week not only ensures their archers are numerous, but skilled as well!
As far as I know, the long bow is not effective against plate, but you are right in saying that is debatable.
You just have to take a look at the prices for armors, warhorses and good weapons at that time. No one would have payed those prices if it wasn't any effective, even if he did swim in money. 
An argument like this I'm a little wary of. "They were used, therefore they were effective." That makes sense on some levels...for example why spears were used, or shields. But knights were important peoples: is it possible they only armored up and rode on large steeds NOT because it was more effective in battle but only because it kept them safer? Perhaps it wasn't very battlefield-effective (and they would have been more effective on foot with the other soldiers) but they were more interested in preserving their own lives, or keeping up the appearance of their importance (mounted on the battlefield)? It's a possibility anyways.
