CHILDREN OF EARTH 
The aliens called 456 (radio frequency number) sent us a message (like in the movie Contact?) with certain instructions - the British gov. had to build a sealed glass room on (top) Floor 13 of the MI5 building in London (this reminded me on daniel's "E-DIN enclosure", a laboratory), with the Masonic checkerboard pavement - black and white squares - in front of it. The alien representative (hitting the glass wall with a tail and emitting dinosaur-like sounds, but speaking in English - through a device, maybe?) was then "beamed" (Breaking News: Pillar of fire descends over London) in it from the sky (I don't remember if an alien ship was seen).
Before the aliens arrived, children all over the Earth suddenly stopped, stood firmly, and started looking towards the sky (Heaven = alien ship?) saying "We are coming... we are coming... we are coming... back" as if hypnotized. This is interesting, because in the final season, there is a big poster on the wall of a waiting room at a Welsh airport, on which it is written "COMING SOON" with a dragon next to it (similar as the one on the Welsh flag).
Basically, the aliens want 10% of children from every country on Earth.
Some interesting quotes: 
PRIME MINISTER GREEN: "We need a middle man. John Forbisher [permanent secretary to the Home Office - civil service] is a good man. And better than that, he's expendable."
Earth representative of the armed forces: "I bring greetings in the name of the US, the People's Republic of China, and the Holy See - the Vatican City, the Russian Federation, for Australia, Japan, Hellenic Republic, Iran, Iraq, Kingdom of Saudi Arabia." [Cocktail of major religions?]
DENISE RILEY: "No, the first responsibility is to protect the best interests of this country, right? Then let’s say it. In a national emergency, a country must plan for the future, and must discriminate between those who are vital to continued stability and those who are not. And now that we’ve established that our ["management level"] kids are exempt, the whole principle of random selection is dead in the water anyway [...]"
DENISE RILEY: "Let me finish. Now look, on the one hand, you’ve got the schools, and I don’t just mean those producing graduates. I mean the pupils that will go on to staff our hospitals, our offices, our factories; the workforce of the future. We need them. Accepted, yes? So, set against that, you’ve got the failing schools; full of the less able, the less socially useful. Those destined to spend a lifetime on benefits, occupying places on the dole queue, and, frankly, the prisons. Now, look, should we treat them equally? God knows we’ve tried and we’ve failed, and now the time comes to choose. And if we can’t identify the lowest achieving ten percent of this country’s children, then what are the school league tables for?"
PRIME MINISTER GREEN: "Then, there we have it. John, you have your criteria. We’ve selected the ten percent."
BRIT. OFFICIAL: "A reduction of the population could possibly, just possibly, if presented in the right way, be seen as good, sir."
If you remember, there was a hype for vaccination in 2009 against the bird flu. In the series, the "Indonesian flu" was mentioned - the aliens gave us the cure in 1964 in exchange for 12 children.FROBISHER: "We need a cover story to explain why the operation is happening, and to encourage participation. So the suggestion is we announce the children will be given some sort of inoculation; a jab to stop them speaking in unison. We stress that there’s no immediate danger, that everyone will be seen in due course. Then when it goes wrong and the children disappear, we blame the aliens. Claiming innocence, we face the music."
RICK YATES: "It is the aliens’ fault, and that’s where the blame should lie, not with us."
FROBISHER: "We play the part of naive dupes rather than willing accomplices."
CAPT. HARKNESS: "There’s a saying here on Earth, a very old, very wise friend of mine taught me it: an injury to one is an injury to all. And when people act according to that philosophy, the human race is the finest species in the Universe."
PRIME MINISTER GREEN: "What now?"
RICK YATES: "Two choices: eigher we go to war against an enemy we can’t beat, or we go to war against our own people, for their own good."
DENISE RILEY: "An injury to one is not an injury to all. We can’t go to war, we have to surrender."