Well, it's true. Some give less than 100% confidence and at a hub like LHR it's almost impossible to sort who in the security queue has come in from where .
If expert profiling were accepted despite all the howls about political correctness, discrimination etc then a large % of the queues could be waved through without further ado and the problem would disappear. It's not all about race, religion etc as there is always the possibility of a wild card and someone being prepared to blow up his or her own family but with the right level of expertise a first rate queue observer can deal with this possibility too. Unfortunately the cargo incidents will probably give the rear end coverers new life.