I find this a little strange.
If, as you say, you keep to the speed limits, why should the location and authenticity of any speed cameras be of any interest to you?
As far as legalities go, I cannot see any law preventing somebody from erecting a fake speed camera on land that they own. The notion (in the Mail article) that a driver who may be distracted by such a device to an extent that causes him to crash, and who may go on to sue the landowner is fanciful. If anything the driver may himself face prosecution for careless driving.
Laser and Radar speed camera detectors are outlawed under the Road Safety Bill of 2004. However as far as I am aware, although this bill has received Royal Assent, many of its provisions have not been enacted and I believe the detection devices are still legal.
You may wish to consider, before getting one, that drivers have been prosecuted for operating devices such as these (and indeed SatNav devices) whilst driving. Although not specifically outlawed in the same was as mobile phones, fiddling with any device in a way which leads the driver not to have full control of his vehicle is an offence.