"Threads" was released on DVD in 2000 by the BBC but was never re-issued. I got my copy from an amazon.co.uk reseller who sells bootleg copies. It is WELL WORTH the shipping to the US. It was originally made in 1983 specially for TV by the BBC and broadcast in big fanfare. I still CANNOT BELIEVE they showed stuff this graphic on prime-time tv back then!
I watched this some months ago and it's probably one of the BEST pieces of sci-fi / drama I have ever seen. The BBC really excelled themselves. It also really shocked me, despite the fact that I knew a lot of what was in it from reading reviews and fansites beforehand. It's designed to do that, that it still has the power to shock is a testament to the power and punch of the script and acting.
What I liked most about it was that it concentrated quite a lot on two things that many post-apocalyptic dramas or sci-fi epics don't: firstly, how the button ended up getting pushed in the first place, and secondly, how all the best-laid contingency plans were a waste of space.
In the movie, a large part of the film is devoted to showing the buildup to nuclear war in the background (news broadcasts, headlines etc) while we see in the foreground the "story" of the young couple getting engaged, buying a flat etc.
It shows an interesting angle on MAD: if you are facing an oponent, and you each draw a weapon, the weapon only has power as a threat display IF one of you believes there is a chance of the other actually firing theirs. If you are both totally convinced the other is never going to fire, you can each end up feeling too confident and upping the ante more and more aggressively until you are so much in each other's faces that one of you feels they HAVE to fire, even if it means death for them too.
So both sides (USA and USSR) behave as if no-one has nukes, and then... well, watch the movie. The public get more and more agitated, but by the time they realise it might actually happen, it's too late... the film was made in 1983 for TV and is in some ways dated, but if you can ignore the period setting it's a really, really good piece of cinema.
There is a lot available on-line, also look up the old 80s "Protect and Survive" material on this site, also contains a link to a comprehensive review of "Threads":
http://www.cybertrn.demon.co.uk/atomic/
I have a morbid fascination with this movie at the moment. I keep on recommending it to every sci-fi fan I know....