This is a serious issue - my mother is a driving instructor, and she also trains people who want to be instructors. Also, one of her best friends is an examiner and her ex was the highest form of examiner possible.
Recently my mum was teaching a guy to become an instructor and he wasn't able to read the number plate from the specified distance. He was a good driver, but he wasn't allowed to progress because of his eyesight.
What you've got to remember is that this isn't just the case of being able to read a number plate, it's essential that you can see on the roads. Say you're on the motorway and there's a crash you can't quite make out. You may not be able to stop in time to prevent a collision. You also have to see what's around you CLEARLY to make sure you can detect hazards, read sign posts, etc.
Note, this is for your safety, and the safety of those around you.