All commentators, whether UK or not, whether ex-professional players or not, call it "unlucky" when a shot hits the post or bar. It is not unlucky. The posts and bar are in the same place in every stadium in the world. And they don't move. So, if you hit the post or bar, your shot is too wide or too high. It is just the same as your shot going outside the post or above the bar. The target is clearly defined. Now, if the goal moved back and forth randomly during the game (sounds sci-fi, eh?) then it would be unlucky if the post happened to move into the path of your shot. But since that doesn't happen, it's not "unlucky", it's just bad finishing.