The only real flaw I found in the movie was the basic economics. I noticed some bits on unemployment but I think that's me looking for it because of reviews I've read. kamalloy didn't really notice that and thought that most of the anti-immigrant sentiment seemed fear based. All I can solidly point to is the occassional "British jobs for Britons" so Fair enough. However, if the world's collapsed than Great Britian is actually likely to have a massive labor shortage. I think the best way to have resolved that would be to downplay the "only Britain survies" aspect. Similarly, it's not really clear why Britain is doing better than everyone else. My bet is that some Brit invented the really cheap video screen technology and they've been coasting on that for years. (Apparently the Britain only thing was an addition. It wasn't that emphasized so maybe I'll just write it off as internal propoganda. But on the whole unwise addition.)
I'll probably read the book, although from what I've heard the movie is better. The book apparently had a heavy pro-parenting slant. I'm taking this from the discussion in comments. The book also didn't have the ethnic minorities angle.
"Pornography and sexual violence on film, on television, in books,in life had increased and become more explicit but less and less in theWest we made love and bred children"
"sex totally divorced from procreation has become almost meaninglessly acrobatic"
Thematically I think Children of Men is a child of the enlightenment. It puts its hope not in government or revolution but in science. It's also overtly playing with a theme of faith and chance. Faith most directly seems to be refering to the idea that we can make this world better. Faith that the human project actually exists. By going with a positive ending, the movie seems to argue that such faith can be justified but that it's not going to be easy.
Anyhow, that's enough to start things off. I'm going to go meet with my boss then make a post office/soundtrack buying run.