It is, I think, to work out how long it takes to go through security in order to work out whether BAA should pay a penalty for excessive waits.

As you identified the timing is somewhat suspect as it does not kick it until you enter the screening area. Hence at T2, all the time spent in the barriers outside is not counted and they have the machines so close to the entrance that they almost always meet their targets and suffer no penalty.

http://www.guardian.co.uk/business/2...irlineindustry
http://www.timesonline.co.uk/tol/new...cle1485290.ece
http://www.telegraph.co.uk/news/ukne...of-misery.html