Sorry to upset you but LHR and some of the surrounding area have some very specific electronic equipment. Number plate recognition systems are prolific around Heathrow, both front and rear readers, so your detector will probably be sensing these emissions while they read your number plate. Also don't forget that some traffic mapping systems do frequently monitor speed of vehicles passing under them, so they can advise users of known traffic blackspots. Also around Egham roundabout you are passing very close to the M25 which has speed cameras in the gantries and several on the road, during the roadworks.