Why would you think they would be radio-controlled?
It wouldn't be infeasible, now, today, as an "art project", to spend £50k, get a warehouse full of them, strap something on the bottom, modify them to fly to preset locations and rip out their radios, drive 10,000 miles out of the way, wait three months to ensure you're home and dry and out of suspicion, and then dial the magic number or set the magic bit on an onion address, that causes them to be powered up, float off through the roof and all attack a different target, or all home in on the same target.
Taking them out would literally require an EMP that knocks out all kinds of things, and you'd never know if you got them all.
With a bit of onboard coding, a solar panel and an Arduino, you could program them to all run off, sit on a random roof for a few weeks and then form a co-ordinated attack at the same time anywhere in the world. Stopping, charging, and then carrying on if they get slowed up or take heavy losses early.
The danger in these things is actually predicted by the mischief that could be caused in the hands of a crop-circle-maker, protestor, artist, or even just kids playing games.
At no point, once powered up, do they need radio to do that job. Even with deliberate GPS takedown (in a matter of hours? Yeah, right), there's nothing stopping them triangulating from cheap 3G sticks which will give you position to a couple of hundred meters on their own, let alone in tandem.
To be honest, the threat of a hostile drone swarm, even from another country, is scary. They could be sitting for years, building up, lying in fields, flying off if anything comes near, and then making a coordinated and almost indefensible attack using any payload they can carry. Even the military were looking at drones that could sit on powerlines and charge up, etc. but a determined hacker with a thousand identical drones which can carry even the smallest of payloads could cause a nuisance and a news story at least, commit terrorism, or prompt a military response if they did it right.
Seriously, there are drones out there with ranges of 7km and flight times in the hours. They ain't cheap but a year ago they didn't exist. Next year, you'll hear of some enthusiast flying them near Heathrow, the year after that they will be in your toy shops.
And if ever there was a tool that it's difficult to determine the source of, especially if you don't use radio, surely that's gotta be one. How many people are arrested when their drones buzz Heathrow, football stadiums, even the Empire State Building? Almost none, because nobody knows who was flying them.