Pretty sure Neil's right – it's an antenna and receiver for a lightning location system, and EA have been operating a lightning detection service in the UK 'for 30 years'.
There will be several similar installations in electrically quiet locations scattered around the country. I guess the box contains power, a small computer, 3 or 4 radio receivers, and a phone line. The direction and intensity of any lightning detected is reported back and triangulated centrally.
Historical analysis of lighting strikes reveals which areas are more at risk of lightning damage than others, which helps Risk Assess maintenance and new build work.
Active monitoring gives early warning of electrical storms and their track and intensity. The service is used by organisations with assets at risk of being struck by lightning – radio masts, cathedrals, nuclear power stations etc. I guess men working on pylon supported power lines are keen not to be caught in a thunderstorm!
Similar loop antenna on the INGESGO website:

Dave