There should be a separate security channel for transfer passengers as their needs can be very different from joining ones. Many transfers are on tight connections sold as such to compete with Amsterdam and Paris in particular, not to mention the Gulf airports. Most,- but not all of course,- joining passengers are in a bit less of a rush , especially as Economy passengers are (needlessly) told to check in at least 2 hours before departure.
Transfer passengers on short connections are though of little interest to the BAA as they have little or no time to spend money in the shops and eateries whereas the earlier the joiners can be cleared through security the more time they have for shopping, eating etc. Hence it pays the BAA to design the system around the joiners not the transfers and if the transfers miss their connections that's OK by the BAA as their dwell/ spending time goes up, often substantially.
Like Bob Ayling who famously said that transfers were probably only worth the price of a cup of coffee to UK Plc, the BAA do not understand that the volumes of transfer passengers justify the frequencies of flights and range of destinations from the airport and that also brings in more joiners and avoids passengers travelling via competing points instead, thus avoiding spending money with both Heathrow and British airlines.