I don't think I've ever shown my passport at security in the UK. Why would I? It's not their job to check my ID, it's just their job to make sure I'm getting on a plane and not taking anything dangerous with me.
Anyway, someone said there should have been a discrepancy at the gate. Why would there have been? The boy had no boarding card and wasn't checked in, even if the gate staff counted all the boarding cards and checked them against the system 100 times they would still never pick up that someone extra was on board as there was no trace of the boy on the flight. Gate staff don't stand and count the passengers as they go through, they just need to match the number of boarding cards against what is shown on the screen and then, if required, match what the cabin crew count on board.