BAA have been fined (yes, and quite substantially) by the DfT for not meeting their security time targets. Alas the fines go to Gordon Brown (who doubtless is never stuck in a queue), not to you and me whose time is wasted.
To overcome this, they came up with the idea of "one bag only". Of course it was presented as being for security, not as a cash-saving exercise for BAA. Nor could they invest in more screeners, because being good business people there is no Return on Investment from that.
There is a management belief that "one bag only" will be their financial salvation.
The pre-screeners (known to some as the "yellow jackets", to others as the "gate lice") are obtained from an agency source. You will all have noticed their only qualification is that they were rejected by McDonalds to be burger flippers. They work to a fixed format with no discretion because that is all they are good for. As with much of aviation security, with a lot of money sloshing around for no good purpose there are always business services which suddenly materialise to help you spend said money.
In a sane world British Airways would be having BAA's guts for garters in the media at every opportunity for screwing up their hub operations. Unfortunately now BA have completely retreated to the BAA hubs at London it is BAA who can call the tune. Do they have to give ALL the gates at T5 to BA ? Only if BA are nice to BAA and their management. Do BA want to have any increase in gate numbers at all ? Only if BA are nice to BAA and their management.