British Airways uses an award chart that is vastly different from most airlines. Most airlines use a region based award chart; which means that the award cost of the ticket is based on which region of the world you depart from and which region you end in. British Airways on the other hand uses a distance based award chart. This means that as long as your flight is under a certain distance threshold, you will pay a fixed amount for the award ticket. If your flight distance is above that threshold, you will pay a higher amount that corresponds to the next distance zone. There are times when region based award charts will better suit your needs and times when distance based award charts will.
A crucial piece of information to keep in mind is that British Airways calculates distance and charges you for each segment separately. This means that you are best off using British Airways Avios on itineraries without a connection. Furthermore, you will get the best value by sticking to flights that fall within zones 1,2, and 3. Here is British Airways’ chart defining each zone and the costs associated with each zone: