So why are people without valid docs not put on the next a/c out from whence they came?
Just think about this for a second. Aircraft unloads at the gate. Pax without documents walks through into immigation hall, with pax off a dozen other flights arriving in this quarter of an hour. It takes an hour or so to line up an interpreter. The pax says he has just got off a plane. He has no idea which airline flew him to the UK, or where from - he just changed planes at the previous airport. After all, he had no need to know because the person whom he paid to get him to the UK escorted him onto the aircraft and ten took all the documents away, didn't he?