Well, it's been some time since I played the english, but if you send him alone to attack york, it should be possible to get odds of 0.8 when the garrison sally, and you shouldn't have any problem routing and killing them. It's a bit tricky to rout them, although it certainly is possible if you charge and destroy the spear milita, then turn back and charge into the heart of the enemy, which should trigger a mass rout.

But I fully believe that england in this game is not made to be chivalrous.... It's so much easier to be evil with them.