Dosent work. There are loads or articles and forums on the subject.
Firstly if ytou are caught or suspected of using it you will be prosecutred, plus if the reverse the picture so it becomes a negative it shows the registration. They have other methods as well for enhancement to get your reg, but of course it is entirely up to you.
And the road angels will be illegal soon too i'm afraid.
I got me a TomTom GO GPS, Thats great cause you can download all the speed cameras from the internet as "Points Of Interest", update them weekly. Then you set an audiable warning when you are within 300 yards of a fixed camera or known mobile camping location. Its like a claxon going off inside your motor, works a treat, not to mention that its impossible to get lost!
It's def worth the £400 odd quid, best driving aid I ever bought.