I'm not really Mr. Extract, but do you still have English-origin pale extracts available? I would go with one of those first. If not, then use a domestic pale extract. You might steep 2 oz of Biscuit malt along with your specialty grains to get some of the flavor. It won't be the same, but it might be more in the ball park. In a brown beer, the darker malts are going to be carrying the flavor more anyway. If it's an English brown recipe, you might see if you can get some brown malt and steep some of that (2-4 oz, maybe).