Almost certainly, the controller you would be talking to would not have access to a map and his radar would not show co-ordinates. If you gave your position as "XX miles southwest of YY" he would look at the radar and probably see you immediately. Also, what GPS co-ords would you use? Lat and Long in deg, min, sec? Lat and Long in WGS84? UK OS grid reference? It would be far too time-consuming for a busy controller to use them.