Not so...I'm no optician but i do not think sunglasses restrict your sight in the same way,if at all,as darkened glass..
Having had some prescription sunglasses made at specsavers (other rival opticians are available ) I was told there were three levels of tint available.
The highest of the three (called class 4?) is illegal to drive with.
"According to the AA: “Filter category 4 lenses only transmit between 3% and 8% of light and are not suitable for driving at any time.
If you have these type of sunglasses, then it is illegal to use them while driving...”