Mostly luck of the draw. Domestic departures require any gate that has the biometric equipment on them. This used to be a very limited number but now extends to all T5A gates (except a few of the A10 bus gates, I think A10a is the only one with the kit) and a few of the B Gates (B44 and B45 off the top of my head but that could be wrong). 90% chance you'll be in T5A but this is no longer a guarantee.