To be legal for road use a visor must comply with British Standard 4110:1999. The Standard means that the visor has been checked for light transmission, amongst other things, and that the light transmission is at least 50%. If the light transmission is below that then you can’t use the visor on the road. You’re probably right in saying that your sunglasses are darker than your visor – an average pair of sunglasses has a light transmission level of about 24% but can be worn for road use even though they let through less than half the amount of light!
