This actually happened to a passenger I was dealing with some years ago. I was escorting him back into baggage reclaim for him to get something he had left in his case and needed for the flight. We went through the Staff Entrance and Security frisked him and found an envelope containg a not insignificant amount of cash. They just called HMRC who asked him a couple of questions and then let him on his way. I assume that the Security people have a duty to report such things to HMRC. Also, HMRC would be at the boarding gates for certain flights, checking passsengers for large amounts of cash. I presume that they have their reasons.