It's not so much the GI rating you need to consider, but the total carbohydrates you consume. Most loaves now seem to put the carbs on the nutritional information as per slice too, so it's easy to work out exactly how much you're having. A normal slice of brown bread is in the region of 15g of carbs per slice. The Burgen bread brettsza mentioned is 11 g carbs per slice (and they're large too), so certainly a saving.
If you have a Lidl near you they do some 'high protein' buns in the fresh bakery section. They're a large triangular dark bun covered in sesame seeds and sold loosely for 39p each. They have a lot of flax seeds in the bread, although it's very light and fluffy inside and they are quite a substantial eat. They work out at just over 10g of carbs each bun but each one is like eating 2 slices of bread - one would be enough for a substantial butty when filled with good stuff - I have half one one toasted occasionally with my scrambled egg for breakfast.