A couple of weeks ago, an awefully "nice" BAA man at security at LHR T3 told me I couldn't take two carry-on bags onto a flight. I had openly displayed these bags and had them weighed, and had the airline's approval to take two bags onboard. I politely told the awefully "nice" BAA man at security that it was none of his business how many bags I took on board - that the airline had approved me carrying two bags, and the BAA's job was to security screen what the airline was prepared to allow me to carry. Enough said - two bags went through with me.
The BAA's job, as provider of screening services to its airline customers, is to screen whatever bags their customers want them to screen, not to impose their own limits. The limits the BAA are attempting to apply are merely a means to reduce the queues (and thus the blame / frustration) that builds up at check points as a result of them often not staffing all of the machines and detectors at peak times.
Andy