Googled "bbc supervolcano" and the second hit was the BBC blurb on the show:
Supervolcano
The beauty of America's Yellowstone National Park masks one of the rarest and most destructive forces on Earth - a supervolcano.
A new two-part BBC factual drama asks: 'What if Yellowstone erupted?'
Parts 1 & 2: 13-14 March, 9pm BBC One
No listing on network or timing for US broadcast there.
Here's another hit from that same google search:
Yellowstone Supervolcano Drama on BBC this Month
By James Wray Mar 4, 2005, 08:55 GMT
which has more detail on the program. It does state that it is co-produced by BBC and Discovery (among others) so I guess that means it's going to be on Discovery.