Make sure that you are fit to drive. You MUST report to the Driver and Vehicle Licensing Agency (DVLA) any health condition likely to affect your driving.
Law RTA 1988 sect 94
Driving when you are tired greatly increases your risk of collision. To minimise this risk
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.
Slow down, and if necessary stop, if you are dazzled by bright sunlight.
At night or in poor visibility, do not use tinted glasses, lenses or visors if they restrict your vision.