I'd guess so long as you never use it (and it is nor obvious) you should be fine, unless the police went over the car with a fine tooth combe looking for problems.
US Law - This is my original take on it, but that is US. :/
Trying to find a legit site about the UK though is proving a pain, loads of ***** sites that say it is fine....but they are ***** so I'm not surprised.
EDIT: Latest source I've found is 2008 and that is a bill not a law.
EDIT:
So yeah, no specific law against them,
but as you are perverting the course of justice it is an offence.
Generally the use of them from the looks of the different sites is that you can use to detect, then drop speed and turn it off. But to be quite honest, why bother? Either don't speed and be fine, or if you are going to speed, just get caught like everyone else it isn't that big a deal.