The Official Highway Code
Rule 92 - Vision
You MUST be able to read a vehicle number plate, in good daylight, from a distance of 20 metres (or 20.5 metres where the old style number plate is used). If you need to wear glasses (or contact lenses) to do this, you MUST wear them at all times while driving. The police have the power to require a driver to undertake an eyesight test.
Laws RTA 1988 sect 96, & MV(DL)R reg 40 & sched 8
Look at the Driving Test Report DL25A as seen above. This is the report examiners will used whilst out on test.
Section 1a Eyesight - is the first thing they'll check when you walk towards your car.
Note that there is only one box under the 'S' for serious fault under this section. If you fail your eyesight check, the test cannot proceed.