There were a lot of stand alone units a few years ago, but they died out with the advent of sat navs, and phone apps, and the latter may be the way to go, as the updates should get pushed to the phone regularly.
But when we talk about stand alone units, are you by any chance refering to the detectors that tell you when you're being tracked by a radar gun, as they are obviously illegal. There was a case recently where the police found their signal was being disrupted by one of the more advanced detectors that scrambled the radar check, and he got the book thrown at him. I could be wrong, but I think the only legal ones nowadays, are those that know the locations, not those that can detect the signal.