I was watching the news in the early hours of the morning as the Gatwick emergency unfolded.
A drone user was explaining how such a thing could be done very cheaply with a couple of off-the-shelf drones for a few hundred pounds. They can be set off from one place but the 'pilot' does not have to remain static, they can move about making them very hard to apprehend. A more sophisticated drone can be pre-programmed to fly a route and off it goes.
The current emergency is not thought to be terror related, but it does not have to be. Any old malcontent, or groups of, could bring a few airports to a standstill with a bit of concerted effort and for not much money, but at phenomenal cost to the wider community and economy.
The only surprise about all this is that it has not happened before.