Both your and Easy10's points are correct of course. I don't mean to suggest that this is an easy fix. All I mean by my comment is that does allow a fairly rapid response to an increased threat. Whether or not that response is adequate is another question entirely.
I work at Heathrow. I believe that there is a bye law in place specifying that only people with a legitimate reason to be at the airport may remain in the concourses. The police are therefore entitled to make any enquiries they deem necessary in order to establish the basis for a person's...