Bu**er! Posted that one by accident! Now I've got to add the rest as another post!
The standard attack should be 1, the defence should be comparable to the best medieval defenders.
Why a barrage unit? Basically, longbowmen were kept at a distance, and softened up the enemy for the knights. The English archers were especially affraid of close quarters battle, as any captured man would most likely be immediately killed rather than ransomed or traded. This was because archers were so effective as battle winning troops that the enemy never wanted to see them fight again. Also archers were lightly armed and armoured for mobility and cost of equipment, and wouldn't have lasted long in a mellee, anyhow.
Incidentally, the 'one fingered salute' was invented by these archers. A compromise by the opposition would be to cut off a captered archers string finger. Before a battle 'intact' archers would taunt the enemy, "Look at us, we can draw bow!".
Why an English UU?
Feudal England passed a law requiring all men to practice archery. This paid off during the hundred years war, when hundreds of ready trained archers were drafted, from peasants to convicts.
Crossbows.
Other countries didn't train archers in such numbers. Crossbows are easy to learn (like rifles), and easy to use. Longbows require a great deal of strength to draw. Winch crossbows, though, were less accurate, and could only fire three times a minute (as opposed to twenty or thirty times for a longbow!). Barrage strength and shield cost should reflect this lower performance.
Do these units work better as firebases, or should they be left as they are?