Boarding a 380 at LHR is an absolute mess. I flew out to JNB last week and it was a disorganised zoo. You couldn't get to the fast track because the area around C62 was just a mass of people. At JNB in April I passed the boarding area for the 380 service back to LHR on my way to get my QF flight to SYD and the area was organised into seat rows, so potentially better organised. If you want hassle free access to fast track, you'll need to get there early.