There was a similar device on sale which was supposed to prevent speed cameras from reading your numberplate, by blasting it with light and overexposing it when the speed camera flashed you. I don't know if they were legal or not, but I suspect they weren't. I think they tried to get round the law by claiming it was an anti-paparazzi device, not anti speed camera.
You have always been able to buy a "paint" which "broke up" the number plate numerals/letters when flashed and also I believe a "Fresnel type lens" which clipped over the number plate supposedly not visible in normal light but again broke up the number plate details when flashed.
The police claimed of course that any attempt to avoid identification was illegal but a bit of a grey area I suspect...? Probably just easier to comply and trickle along with everybody else!
All this numberplate talk reminds me of some plum who'd read that rubbing lemon juice on numberplates can aid in masking the details from speed cameras. Fortunately for the police and the bank he robbed at gunpoint, rubbing lemon juice all over his face did absolutely nothing when being filmed with conventional CCTV.