In the US buttermilk is a cultured milk like khefir. Originally it was whey (thus the name) that had soured a bit. The main reason for using it is that if you limit the leavening to baking soda, you need an acid to react with the soda. Otherwise you have hard, crispy little objects that can be substituted for hockey pucks. Thus, the addition of sour milk (buttermilk). Since we often don't have buttermilk on hand - a large minority actually drink it - there are numerous doges. Probably the best, since it adds just a hint of flavor, is sour the milk with a tablespoon or so of lemon juice. Simply add the juice to the milk and let it stand for a few minutes until the milk has begun to thicken. Also, a good many of us biscuit makers toss in a teaspoon of sugar to enhance the flavour a bit (too much makes the biscuits sweet) and to enhance browning. Acidic doughs don't brown well and unbrowned biscuits don't taste as good.