Because the onward flight is shorthaul it may not work very well. While the longhauls are generally well advertised within B and C ahead of time, the shorthaul services generally need to actually arrive first before the departure gate can be advertised, just in case there is a last minute switch. You could just hang around T5C just on the off chance, or since you have lounge access, go to T5B slowly and try Flight Connections there, but I'm not overly optimistic.