In modern political parlance "I take full responsibility" has, ironically, become a way of avoiding responsibilty altogether. It's a phrase without consequences. When I try to imagine Cameron, or indeed any front-bencher (with one or two honorable exceptions such as Hilary Benn) resigning Lord-Carrington style because of some gross lapse of competence or judgement, I find I cannot do it. However, the phrase retains the power to close down discussion. Once someone has said "I take full responsibility" there seems to be nowhere else for a questioner to go. It's a clever tactic.
Since then, Cameron has taken full responsibility on a regular basis. In January 2012 he declared of the faltering economy: "The government takes absolute responsibility for everything that happens in our economy, and I take responsibility for that"; and last week he took "full and personal responsibility" for the debacle of the Syria vote. Yesterday, after the National Audit Office's damning report on the implementation of Universal Credit, Iain Duncan Smith took up the cry, insisting that he took "full responsibility for the delivery", while in the next breath blaming civil servants for all the mistakes. IDS isn't as slick as Cameron, and he rather muffed the trick by exposing its dishonesty too plainly to view. Nevertheless, he is still in post and no doubt retains full responsibility's Siamese twin, the Prime Minister's "full confidence".