Here in the U.S., baking powder is totally different from baking soda. Baking soda is the same as bicarbonate of soda…
One site explains it this way: "...baking soda is an alkaline, and when you mix in something acidic, like vinegar, it will release gas. The key here is that baking soda needs some sort of acid to activate the reaction. So it will work in recipes that include acidic ingredients like buttermilk, sour cream, lemon juice, yogurt and so on. Molasses is also acidic, and so, believe it or not, is honey. So any of these ingredients would activate the baking soda. But if you were to try to substitute baking soda for baking powder in a recipe where no acidic ingredient is present, there will be no release of gas and the dough won't rise.
Baking powder, on the other hand, is nothing more than baking soda with some sort of acidic compound (different brands of baking powder use different compounds) already included. The baking soda and the acidic compound won't react together until they are moistened, which causes the two chemicals to mix. So-called "double-acting" baking powder is also activated by the heat of the oven or griddle, and thus has greater leavening powers…"