I’m flying in BA J to London and want to know what my best lounge option is. I also have Amex Plat. Skimming through this, it sounds like I can get into American Flagship (is this in T4? Connected airside to TBIT?)?

Looking for complimentary champagne and cocktails with good food. Preferably not too crowded, but hopefully all the lounges are not too crowded in the middle of the week.