It would be fairly easy for them to climb over the steel entrance gates, get through the hedge, or maybe they came over the fields surrounding the airfield.
If there is anything to be thankful for, it is that they appear not to have vandalised the aircraft parked between WAM and the "new" hangars, or the aircraft in the back field parking area.
Hopefully these lads can be identified and prosecuted, but I can`t see any way that you can protect anything the size of most airfields from anybody intent on breaking in.
Best wishes to all my old friends at EGHP, and I will try to get down there soon to see if any help is needed.