If programmes are encoded in dolby surround (can only think of the simpsons and malcolm in the middle off the top of my head) if you set your DAVS-b100 to pro logic 2 you should get surround sound.
Otherwise personally i would just leave it on that and it will do its best to make surround sound out of the 2ch signal.
AFAIK, there are no ntl boxes that have an optical/coaxial output so you cannot get DD5.1 from anything full stop and anyway, i think the sky movies channels are the only ones that broadcast in dd5.1.
For the UK, I'm guessing with the arrival of HDTV we may start to see programmes broadcast in DD happen
