Yes you are right, but that is for the new style number plates post 2001. 20.5 meters is still in force for number plates pre 2001 which is why it still sticks in my mind.
20 meters is about 5 car lengths.
Always bear in mind that if the Police tell you they want to do an eyesight test then they are acting illegally. There has been case law over the years where a Copper has required an eyesight test and the Ophthalmic industry kicked up because only a qualified optician is qualified to do a test.
Small point maybe, but important to them.
A copper is allowed however to check your ability to read a number plate at the prescribed distance, and if failed then the vehicle can be seized (for the drivers and public safety) and reported for driving with uncorrected vision.