It is not the reason. As previously stated the reason is that all pax taking off from the UK have to be cleared by UK security. At the current time the minimum standard that ALL the world's pax handling airports must adhere to are set out by ICAO. In Europe the standards are further refined and improved by ECAC, and in the UK the standard - tighter again - is set by the UK Government. Therefore at world hub airports like LHR it is quite likely that a significant percentage of transiting pax could have boarded their flight in a country complying to the ICAO standard, which is not currently as high as the UK Standard. Therefore the rule - all pax taking off from a UK airport must have been processed by UK security. All transit bags will go through the HBS system at LHR.
Stand 323 is on Pier 6, 322 is on the end of Pier 7. There is a huge amount of visible security, and an equally huge amount of invisible securty on Pier 7, which is in place until Pier 7 is demolished and replaced in the near future.
For DfT rules, try the DfT website.
Security in T3 cannot wait for the DfT to relax rules on the number of bags permitted through security. Security queues only became a regular feature after the Liquid threat last August. Believe me, BAA want nothing more than for pax to be speeded into the Departure lounges as quickly as possible. No money is made out of pax in queues. Infact, you would be surprised at the amount of money T3 refunds airlines because of queue lengths at security. Ferrovial have set a target of 5 min queue time.
If you find the current security process difficult, complain to your MP, complain to the DfT, but don't assume that any part of the security process is at the request of BAA. BAA are legally required to comply with the UK governments regulations and LHR is constantly monitored by the DfT to ensure this happens.
If aviation security in the UK is still not to your liking then take the Eurostar to Paris and make your future trips through CDG.