They could follow any one of the drones.
20 drones placed around the perimeter, timed to take off at half hour intervals and fly a pre-programmed route across the runways. A few others timed to take off a couple of hours after the last ones to increase uncertainty.
No one there, no tracing a drone ‘back to base’. Lots of individual drones taking off from different locations – tops of buildings, roofs of containers etc etc – can’t be seen from the ground (and not at all at night).
Do that at every major UK airport in succession and you’ve disrupted all flights for a week.
total cost, a few thousand pounds.