Its true, there should be a permanent incentive supply run to Gallic occupied areas to encourage more trading in exposed zones (as well as hauling plundered goods back to Gallia). I know there are decent ones now but a big one would definitely draw in a more permanent presence. If London is about to be sieged, there should be a high value cargo run to both New London (from Liberty or the Omegas) and another for the Gallic side to their closest frontline base.