If you want to play the odds, head to Galleries North as domestics often board at the low numbered A gates (or A4, if parked on a remote stand) at the northern end of the terminal. Though that rule is a lot less true that it once was, with the expansion of biometric gates. On the day, you could use FR24 to see what aircraft is operating your flight and track it inbound to LHR to see where it parks, and whether a switch to Galleries South is warranted. Though I do think that's overthinking things a bit. The walking distance between the North and South lounges must be 3-4 minutes max.

Any chance you were at the B gates Galleries lounge?