Not as simple as that. Many longhaul BA flights have Premier Economy (which they call World Traveler Plus) in between Economy and Business. On those flights, you CANNOT upgrade from Economy -> Business. You can only upgrade from Economy -> Premier Economy, or from Premier Economy -> Business.

Also on some airlines (I don't remember if this applies on BA), it's a different upgrade code depending on whether you're upgrading form a discount fare or from a full fare.