I would suspect prank. And I would take reports of them being 'large, professional' type devices with a pinch of salt for now.
But.....As an act of state sponsored disruption it's pretty low risk for the effect. Send in some low rent heavies with off the shelf drones to repeatedly buzz large airports causing millions of pounds worth of disruption. If your guys get caught they'll probably just get a summons by which time they are safely back in the motherland. If they do end up in the clink for a short while I'm sure the offer of some adequate compensation will make their time go past fast enough.
On a long term basis this is cheap, effective and and as said low risk to state and operative. Much like Facebook or Twitter trolling.
If it's not the Russians or Norks this time I'm sure they are taking notes on the respons to a couple of drones.