Q404: I have seen some spray that can cover my registration number, is it legal?
No it is not legal.
Any interference with the number plate to make it less easily distinguishable to the eye or would impair the making of a true photographic image is illegal. Although the spray does interfere with flash photography many of the speed cameras use Automatic Number Plate Recognition (ANPR) that uses infrared radiation making the spray ineffective.
You could be liable for a substantial fine if you are found guilty of using such a spray.