Now that I've recovered from marathoning TW, my thoughts (I'm echoing other people, mostly, but the thoughts are all my own work so ner):
The entire 'mainstream' audience in the UK seems to have thought it was the best thing ever. I didn't think that, but I liked it a lot more than some people here clearly did. In particular, I didn't hate Jack. Given that this was all about sacrificing other people's children and refusing to take personal responsibility - especially the government scenes, with the prime minister and in the cabinet room - I can see why he thought he was doing the right thing. He was entirely wrong, as it wasn't his own child he was sacrificing. But I think he thought he couldn't do anything else. His flawed humanity in trying to make up for the past and only making things worse was tragic.
On the other hand, while I can sort of see why they killed off Ianto, for several plot reasons, I didn't like it. I *did* sob at the TV more than I possibly ever have for anything on the telly including BtVS. But I didn't like it, with hindsight anyway, and I don't think it was worth the plot stuff they used it for (mainly Jack's stuff).
Vonnie K., I agree that the cabinet room scenes were utterly chilling. (Not least because they strongly echo the way that the British government has been talking about its economically 'inactive' citizens in recent years.) The themes of corruption and the inability to accept social responsibility worked really, really well for me.
bonny fides, you said "456, from beginning to end, has been a total cock-up." Although I didn't think this while watching it, having since given it some thought, I'm with you. Earth basically surrendering, and agreeing to give up 10% of its children, without knowing what they'll be used for, and without even a basic investigation into who this race is and what they're actually capable of (what they *say* they're capable of not necessarily being true - as my Girl said, it was a protection racket)? And that was the tip of the iceburg. That said, I did enjoy the effect of never seeing the aliens. Imagination is always creepier than CGI.
le nubian, there was a brief discussion in my house over whether Jack might have had many more children - we assumed that would be the case, given his age (and his, um, activities). I wonder whether they cut out a reference to that from the script. It seems odd for 'Declassified' to bring up the concept out of nowhere.
In relation to the question of how the British do sci-fi, I can see where Anne W.'s friend was reminded of Blakes 7 etc, and maybe that's why it was generally liked over here (it'll be interesting to compare reactions generally when more people have seen it on BBCA). Although there's a fair amount of that in US sci-fi too - Battlestar's recent ending comes to mind, although not in exactly the same vein, as does much of Tim Minear and Joss Whedon's work.