The movie brings up allot of interesting points, but I do agree that it's politically biased to an extent. The claims about Katrina and the environment are not scientific fact, they're just theory and shouldn't be pushed as givens for example.
There are many such "theories" that are not proven yet, and mixing them with the actual real evidence only brings weaknesses to the strong case being made. We have a theory that pollution on the one side of the planet causes instabilities in the climate on the opposite side, so some speculate that China's pollution levels are causing unstable weather and more constant hurricanes on our end. This is a theory that's not been proven, if someone uses it in a documentary, it risks turning that into sensationalism.
I can understand UK's reluctance if that's what they're worried about. Perhaps they saw a few things in there that went too far for a supposedly rational scientific documentary. That doesn't detract from the reality of global warming though of course, but something more to the point is not a bad idea... might be boring to watch the alternatives for those school kids though, usually those are not designed for populations with short attention spans ;) Obviously made for local consumption and not the already "more educated on the topic" industrialized world abroad.
My only worry is that this was not done out of scientific concerns, but simply due to opposing political idiocies, lets hope not.