Oh dear - I was expecting it to work independently of the camera. The point is, there are times when you need the torch to see the barcode properly before you hit the scan button.
Gordon has found more information (see link). The requirement is for the camera and the torch to work from the same app, but I'm not sure anyone has made an extension for that. You could try the custom camera extension. If it does not have the torch feature, perhaps you could request it.