Fuel surcharges and exorbitant mileage costs is what's wrong with BA miles. Burn them if you have them, but stay away from them in the future.
US fly their own metal to GIG and you can get an off-peak award to South America using US miles (xfer from SPG) It is 60K for Envoy off peak (this is just off the top of my head, I could be wrong so please correct me).
You can also fly United metal without transferring miles to United, you can transfer your SPG to either Aeroplan or US Airways and book the same flights on UA metal.