You know the law. So it's up to you. I used a black visor on an arai race helmet, in the summer, but it was a pain when the clouds came out. I got a flipdown sunglass type thing, which worked much better. One of the RS helmets = schuberth copy. Most comfy lid I've had, with bluetooth as well.
But to be honest, the best solution I found was the (can't remember the name) stick on, uv activated jobby that darkens to a blue color when the sun breaks, and fades to a slight smoked tint when it doesnt. I can't remember the name as I had a new visor sent over by my folks and told them to go to crossans in newry and just ask for a visor for whatever agv it was, and stick that thing on that goes dark!!
Dark visors are just a pain in the UK. Just no guarantee of any Sun, never mind too much!!!!