They are legal
"Camera detectors are legal for use on UK roads. A few years ago there was talk that detectors with radar and laser detection may become illegal under proposed legislation, but this ban didn't take place - meaning that both laser/radar detectors and GPS safety cameras are still legal in the UK.
Always remember that laws are different if you're planning on driving abroad, and that in most EU countries laser/radar and GPS safety cameras are illegal. Speed camera detectors are not legal in the Republic of Ireland."
Can be bought in Halfords -
http://www.halfords.com/technology/speed-camera-detectors/speed-camera-detectors/snooper-4zero-elite
However like mentioned by others I would much rather rely on a satnav to tell me there are speed cameras on a road. Also if you are not speeding you have nothing to worry about.