Q404: I have seen some spray that can cover my registration number, is it legal?

No, it's against the law.

Any interference with the number plate to make it less easily distinguishable to the eye or which would impair the making of a true photographic image is against the law.

Although the spray does interfere with flash photography, many speed cameras have Automatic Number Plate Recognition (ANPR) technology that uses infrared, thereby making the spray ineffective.

You could be liable for a substantial fine if you are found guilty of using such a spray.