Why 6? Supposing there is a winner in the other game (US or Iran getting to 5/6pts). If Wales wins by 3 they are up to + 1 in GD and England goes down to +1. What happens then?England played not to lose as opposed to playing to win which is understandable. Unless Wales beat them by 6 or more goals in the final match they are through.
Edit: Got it. Best attack. So Wales needs a 4-0 win in this scenario.