The spray is not visible so it is still legal.

The fanatical attentiveness on speed has produced a nation of nervous
drivers whose confidence appears to have been destroyed. Nobody wants
to overtake these days and, if overtaken, most drivers react with
annoyance. Skill, judgment and anticipation have gone out of the window
and our roads are more dangerous as a result.

And I wish for that raison d'=EAtre that I could spray everyone's
license plate with a photoblocker spray http://www.photoblocker.com/
and get the whole thing as the crow flies. Everybody will drive with
self-belief and there will be fewer accidents.