Question1: What does "opposite number" mean?
Players have numbers. Each playing position has a number.
Your opposite number is the one on the other team that matches you.
Sorry. not a sporting person so don;t know whether it means the one with the same number, or whether it is the one who stands opposite you, opposes you. As I imaging in a left-right sense they are not mirrored but turned to make the other side's numbers.
Question2: Could you tell me "England can eclipse even the heights they reached in a superb semi-final victory over defending champions..." in easy English, please?
"England will be able to do better than (can eclipse) even the great game (heights they reached) they had when they won the semi-fnal ( in a superb semi-final victory ) against (over) the current champions (defending champions...)