I kind of think there's not an equivalent- British malts offer their own range of flavors. You have pale malts, mild malts, amber malts, and brown malts, which give you quite a bit of flexibility, but none of them matches up neatly with munich. I'd think some brown malt would be good to get a similar effect, but not in a 1:1 substitution- I think Jamil uses half a pound in his brown porter, but I don't have that recipe on hand. So mostly maris otter, with a bit of brown malt will give you something authentically english in character, if different than using munich.