There have been numerous thefts from military armouries over the years (an average of 56 each year). SA80s have turned up in the hands of the IRA, drug dealers, white supremacists etc. There was a fellow a couple of years ago who was otherwise a regular, law abiding guy who murdered a man he believed had been abusing his niece and when police searched his home they found, besides the man's body, some unregistered rifles and shotguns - one of which was an SA80.
There were also some semi-auto versions made for the commercial market which could have gone walkabout (also the straight-pull cadet versions that were sold commercially after the semi-auto ban), but much more likely it's a stolen army gun.
As for 'preparing for something' from the story I understood that the people who found the guns own/live on the farm where they were found. It seems unlikely someone would leave a cache of illegal guns on someone else's farm where they would be found like that. It also says they were found wrapped in a blanket - I suppose criminals often are not the brightest bunch but anyone with half a brain would take measures to protect the guns from rusting if they intended to come back for them. So I'd assume they've just been dumped in order to get rid of them rather than risk turning them into the police. Makes you wonder why they wouldn't chuck them in the sea though!
If you do want to prepare for a 'vote from the rooftops' situation then
this guide may be helpful.