Link below is to some photos of the exterior Flagship check-in area. I was there last week and remember it only being for J and F pax, but am not 100% sure of that.

http://www.wildmanbugby.co.uk/gallery.htm

More important than check-in are the lounges at T3 that AA PLT will get you into. Go early and tour them all.

AA AAdmirals club is under construction and is noisy and looks like a construction site.
BA Gallaries has food, is new and is MUCH nicer.
JL lounge is quiet and opens at 1:30pm.
CX, leave until last for Dim sum and a shower right before boarding.

If you have Priority pass there are also the Kuwait Oasis and Servisair lounges. I stayed at the LHR Hilton the night before, so that made 7 lounges at LHR that morning/afternoon. Just make sure you have your PLT card with you.