You'll probably get responses from people more knowledgable than me, but with my Ford Focus I bought the stick-on deflectors, found they did nothing and set the beam adjustment low. No problems. At least the local police might think I had made an effort, I thought.

On return home, I left the deflectors in place; as I said, they seem to do nothing. No problem with MOT. And handy for the occasional return to the continent.

Do you actually mean the prismatic deflectors, or the mask templates?

I used to use the latter when we went to France every year, back in the 80s. I saw as many of them on other cars fitted in completely the wrong place than correctly.

What you are trying to block off is the light that goes high on to the left verge when you dip. That light comes from the opposite, bottom half of the reflector. Once I had figured it out I started making my own with black insulating tape!

Having said all that, it probably doesn't work so straightforwardly with some of today's headlamps - the odd shapes, and the projector types, which would need a prismatic deflector

I doubt if the flics will be at all bothered, especially in summertime. However, if you are worried, just apply a small long thin black triangle made of duct/insulating tape somewhere on the perspex....