Here's the depressing thing
Let's say there were no drones. If there weren't, this goes beyond incompetence. We've demonstrated as clearly as possible that you can close down a major airport for days with no risk to your own life or liberty and using less than a grand's worth of gear, the sort of gear you can buy innocuously from any hobby shop or off t'internet. If terrorists weren't aware of that before, they are now.
My plan for shutting down Heathrow: buy/build three or four drones - as many as you can afford. Not off-the-shelf geofenced models - Arduino DIY jobs, decent sized, well lit. Program each one to lift off, fly a couple of miles at top speed, then loiter at about 200 feet for five minutes, then fly to the nearest large body of water and ditch themselves. You can test this behaviour anywhere, to your heart's content, until you get it working just right, because unlike, say, a bomb, testing a drone doesn't look suspicious or draw the attention of plod (obviously when testing, ditch into a nice field, not a pond). So you get the "fly,loiter, escape, ditch" behaviour right... drive past the perimeter of the airport in a van. You don't even need to get that close - a couple of miles is fine, a drone can cover that in two minutes or less. Open the roof, launch a drone, drive off. Wait a few hours until the airport is on high alert and flights have stopped. Drive to somewhere else, launch a drone, drive off. Now the authorities know there definitely IS a drone, and it's not mass hysteria. They spend much of their resources trying to track where the control signal is coming from and looking for the pilot - fruitlessly, because it's entirely autonomous. Rinse and repeat every day or two at unpredictable intervals until you run out of drones, and then just go home to Poland or wherever you bought the parts from (you didn't get them sent to a UK address, obvs). Chances of you ever being connected with the event: zero. The only possible way to get a witness is if someone see the drone exit your van and connects that with what happens subsequently. Don't launch where there are witnesses - you only need a few seconds.
Cost to the drone builder :less than ten grand. Cost to the police to investigate: not less than a million. Cost to Heathrow and the travel industry generally: not less than a hundred million.
The problem with this kind of attack is there's no way to stop it that doesn't interfere with the airport's ability to function. Any terrorist cell that can afford an AK47 or a trip to Pakistan or Utah "to train" can afford a drone, and a drone attack is much, much safer and has a higher chance of doing actual economic damage. You can't interfere with the command and control because it's all on board, you can't catch the pilot because there isn't one. You could conceivably try to shoot one down, but they're cheap enough that you can just keep throwing them up there - how long can you keep playing whack-a-drone?
Demonstrating that this is a viable plan is, IMO, the worst thing the police have done here.