NO look at the police website is says
Question
Q404
I have seen some spray that can cover my registration number, is it legal?
Answer
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.
http://www.askthe.police.uk/content/Q404.htm
If a speed camera has ANPR then your getting a fine anyway no matter how much spray you use
Also some speed camers have a standard CCTV camers next to them so your number plate is always on film so you get caught for speeding and using the spray 2 crimes in 1
you plate might not show up on the speed camera photo but will do on the CCTV