In general you need to use the lounge in the Terminal from which your flight departs. For OW Emeralds the lounges available in each terminal are as follows (best, in my opinion, in bold):

T1: BA F (with Molton Brown Spa), BA J, EI lounge (dire)

T2: IB & AY lounges presumably (no personal experience)

T3: CX F, AA Flagship, CX J, AA Admiral's Club (I am not as anti the Flagship Lounge as some - maybe because I always seem to be there at quiet times; I find the chairs more comfortable to sit in than the square design in CX lounges)

T4: BA Concorde Room (only if in F, excluding op-ups), BA F (essentially the BA Gold and OW Emerald lounge), BA J * 2