I did this a few days ago. Went up to the ticket desk in the GC lounge about 20 mins before the flight and they offloaded me, this only took a few minutes. They said that there is a security/escort visit to the CS desk in the main terminal hourly and on the hour, which was only 5 mins away for me, so I just went straight there. I had a little wait 10-15 mins because there were two other people leaving as well. She then took us through to passport control.
So if you are leaving just before the hour then you could go to the desk, otherwise just take the train.