Longhaul can be from A, B or C, and you won't know until 80 minutes or so before departure, unless you get a tip off from lounge or check in staff before then. The A380s are almost certain to leave from C, but I'm sure there will be boarding from A or B at some point, even if it is a one off. So all you can plan to be in South Lounges Complex, and all parts of T5 are within 15 minutes walk from there anyway.