As far as i can acertain from my lmited understanding of vehicle law (I often defer to traffic Officers on this), unless the lights are factory tinted and certified road legal (E marked as such) then it's not allowed.
Some try flyeye or lamin-x treatments as they're not permanent, if you did a permanent spray on tint and got tugged for it then you would be obliged to remove tinting there and then or else the vehicle would be taken off the road until rectified to the satisfaction of a VOSA Inspector.
I was gonna comment on this when I saw it earlier but I thought I'd leave it for our resident law man to give his professional opinion instead ;)
As Clive says if you got tugged by the black rats they would have you remove it at the road side, which if it's sprayed on I don't think you'd be able to scrape it off so your motor would be taken off the road. So it's really not worth the risk basically lol.
Again as Clive says, fly eye film could be an option. I have used it myself on my headlights n tail lights, not sure if i'd recommend it as it can be tricky to apply, nowhere near as easy as they make it out to be.
And it does cut out a fair bit off light, more than they claim it does anyway as my usually very bright osram cool blues looked like they had been neutered as they was quite dull with it fitted. I've since replaced my head lights with some projector headlights with black inners, these are very bright :)
Also it doesn't weather very well, I recently took it off my tail lights cause there was a fair few big patches that had broke off. Thats another annoying thing, they claim its reusable n easy to remove, it isn't. It comes off in little pieces and takes ages.
One good thing i can say about it though is my motor passed its MOT beautifully with it fitted with not one advisory
don't you already have black head light units anyway mate? or arent they dark enough for you still?
you could get away with fly eye n from a distance it will look alright. just up close it looks like you've wrapped em in your missus's tights lmao
but just be prepared for reduced light output n a game n a half if you decide to take it off
but yeah the solid tint films will be a definite no no