It looks like they were only held for a couple of hours and released. Not that I think this is right, but they are flying with lots of wires/electronics/batteries.
It's crazy how the whole movie plot driven security industry has been able to associate naked electronics with bomb making. A friend was commenting last night that the bare PCB of the Raspberry Pi could be seen as dangerous/threatening, and I was speculating about how much trouble I'd get in if I tried to take Arduino based home made toys on a flight.
Friend of mine had a kind home-made led flashlight (small PCB and gangly wires). Friend was pulled aside for questioning and demonstrated its 'torch' capability and then let go thru. The delay was about 20 minutes though. It is silly, but if they've not studied any (even basic breadboarding) electronics, then anything out of the ordinary like that will look 'weird' and arouse caution.
Honestly, there are people at the border whose entire job is to look for suspicious items. (It's not like the threat of a bomb attack in London is a purely hypothetical one, http://en.wikipedia.org/wiki/List_of_terrorist_incidents_in_... ). If you're gonna carry a suitcase full of weird-looking homemade electronics across the border, then yeah, you're probably gonna wind up answering some questions about 'em. In the end, they were correctly determined to be no threat and free to go with their drones.
Also they were coming directly from Dublin to London. Anyone with a knowledge of history should understand the significance of that. Sad but true. Security staff are paid to be paranoid.
I don't think this story has anything to do with Drones per se. I think the same phenomena would be triggered by any suitcases full of electronics.