And what happens when other nations decide that they want to pre-control aircraft at overseas hubs? Are we going to have a dozen different security screens operating at Heathrow, made up of people from a dozen different services with different training, equipment, priorities, institutional culture, obeying or not a dozen different national laws and a dozen different bilateral agreements with the UK, looking for terrorists defined a dozen different ways by a dozen different methods?
I suggested a British security control in the US as a response to the TSA's accident prone, jumpy, low quality performance - I thought at the time that it was a counsel of despair because it would lead to the "proliferating policemen problem" described above. I still think so.