The reason the BAA staff police this sort of queue jumping is that it is rare for all lines to operate a “shoes off policy” at the same time.
Consequently, it might be possible for a wily terrorist, once he/she has sussed which lines are “shoes off” to simply opt into a line where the shoe carnival was not in operation and sneak through unnoticed.
Similar issue might apply if they “knew” one of the BAA people doing the pat down.
It is a justifiable security measure, though would be much easier to know in advance that once you have selected a queue, you must not switch queues.
Thje concept of Fast Track is lost on those responsible for running T5.
BAA blame BA for only paying for one Fast Track scanner, when in fact there should be proportionately more scanners at the end of the Fast Track queue to ensure there is little or no wait for Premium Passengers.
Usual pathetic childish stand-off between greedy BAA and BA’s decimated management, who seem unable to grapple with the problem.
All this when pax numbers are down 10%, giving no capacity excuse for slow processing.
Result: the security waits at T5 are actually getting worse than they were late in 2008, and passengers suffer.