Other legitimate users of low level airspace are the helicopter operators on gas pipeline surveillance flights. Their job is to fly the pipelines routes regularly to see that no farmer/builder is using a JCB or suchlike which might damage the pipelines.
If they see such a situation, they must land and inform the farmer/builder of the dangers of breaching a pipeline.
These helicopters carry a pilot and observer, and some years ago, when I was involved in such activities, we had regular meetings between the gas companies and the RAF to try to ensure that they stayed apart. Our plan was to put lightweight transponders in the helicopters and the FJs would be able to "see" them on their radar. Sounds like it didn't happen.