What is the minimum distance you must be able to read a vehicle number plate to check your eyesight meets the legal standard?

Why?

Rule 92 of The Highway Code says you must be able to read a number plate (in good daylight) from 20 metres, wearing glasses or contact lenses if you need them. If you can't, you must not drive until you've had your eyesight corrected, and you should always carry a spare pair of glasses if you rely on them.

Highway Code rule 92