For a porter you could steep some chocolate malt and some black malt. Use these sparingly as they can be over powering, say 1/2lb each.
For a stout just steep 3/4-1lb of roasted barley.
For a brown add some mid to dark crystal malt, English brown malt, and some chocolate malt. Maybe 1/2 lb each as well.
I've confess I've never done what you're proposing (I brew AG) but it should work and provide an English flair.