You will be departing from Terminal 5.

There are three lounges you can access, Galleries Club North, South and T5B.

Your flight is more likely to depart from T5B or T5C, but you cannot be sure of that till 60-90mins before your flight.

Both Galleries Club North and South are in T5A. They are all very similar, but T5B is much quieter (as the gates aren't announced till late...). South perhaps a bit quieter/less chaotic than North.