The other issue at LHR is that the person scanning doesn't see the bags as they come out.

One of the more sensible features I've recently noticed at US airports is that the person on the screen sits by the bags as they exit the x-ray tunnel, so if they see something suspect on the screen, they can have a glance at what it actually is, before deciding whether it needs to be searched or not. Quite often they'll have a quick look at items, realise they're harmless, and let the bag go.

At LHR, the person behind the screen is totally separate from the belt, so they have to direct anything that looks vaguely dodgy or unidentifiable down the 'search' channel, where the owner waits (often for some time) for the entire bag to be searched and then re-screened.