The BBC doesn't live or die by ratings like other networks, commissioning a second season to resolve the story and spread the production costs out further isn't outside the realms of possibility.
Actually it does, it just uses different language - the BBC talks about
reach and
intended audience. If a show is conceived for a prime time audience on BBC1 and has the budget to match, it has to hit certain ratings or it seen as failing to reach a mainstream audience.
There are also politics at play, if the BBC is playing fantastic sums of money for shows that have low viewing figures, it gives more ammo to the right-wingers who want to bring it down.
A show that is moved from a Prime-time BBC1 slot to the sunday night graveyard by the controller of the BBC
during it's run is dead. Even if it came back (which will not happen), there is no way it would have the budget to be filmed in South Africa or have the effects it does or most of the cast. It would be an entirely different show.