You can't be (successfully) prosecuted.
Why not?
Because you are NOT warning other cars of the presence of a speed camera ...
... you are warning them that they are entering an area in which they should drive more slowly ...
(which is the supposed justification for speed cameras)
So, you are not "perverting the course of justice" ...
You are HELPING the Police to achieve their stated aim of reducing traffic speed.
And that, Your Worships, is the case for the Defence. In my submission, there is no case to answer, and the case should be dismissed with costs for the Defence.