This map shows the locations of Air Traffic Radar installations across the UK.
Data from Ofcom: http://stakeholders.ofcom.org.uk/binaries/spectrum/clearance-coexistence/Protected_radar.pdf (Dated Sept 2014) and http://www.nats.aero/discover/10-year-replacement-of-uk-radar-network-complete/
Each radar head is shown as having an arbitrary 40 mile radius of coverage. This is a reasonable assumption for coverage of GA traffic at low altitudes, but is obviously affected by factors such as terrain and the height of the radar. Clicking on the radar location gives the option to calculate more accurate coverage for the site.