Interesting scenario and one that BA or Heathrow management have not learned from : after all the issues with snow and previously high winds: aircraft waiting after landing to offload maybe up to 4 hours ?? incredible. As a result of previous instances and before I left LHR i drafted a contingency OSI for passenger offloading onto taxiways therefore enabling pax to get in the terminal. Aircraft would then taxi to base to park which was always full of available space. This was available for all terminals not just T5 with specific locations specific processes to achieve. During the snow debacle we and offloaded 10 aircraft in 90 minutes that had been waiting for up to 4 hours because of no available stands. This resulted in the draft process being drawn up with ATC and BA agreeing. looks like its now in the bin somewhere !! This is when experience counts with ops guys who can think on their feet and do not have to rely on a black or white procedure!!!!!!