Providing the route isn’t a monopoly, BA (or any other carrier for that matter) can charge whatever they believe that particular route will sustain. Whether this is by surcharges or direct fares is really only down to marketing strategy.
Air travel is no different to any other business – maximise the returns for the shareholders. After all, this is exactly what the oil companies are doing, only they are also lining the coffers of the exchequer as well!!!.