The actor who plays John Frobisher in Children of Earth, Peter Capaldi, also portrayed Caecillius in the Doctor Who episode “The Fires of Pompeii.” That character and his family were saved from the eruption of Vesuvius by the Doctor, at Donna’s urging. RTD has said that it amuses him to believe that Frobisher is a descendant of Caecillius (which means that the Doctor saved the family in A.D. 79, only for the line to die out in the 21st century).

Meanwhile, remember Harriet Jones, who was Prime Minister (and appeared to have a good working relationship with Jack Harkness) until the Doctor took issue with her treatment of the Sontarans? Granted, the Master might have deposed her anyway — though he only managed to get to Earth via the Doctor’s TARDIS...

In other words, the Doctor’s actions played no small part in ensuring that when the 456 arrived, two key figures in power, the Prime Minister and Jack’s liaison with the Government, were men willing to destroy Torchwood and sell out the people they were supposedly sworn to protect.