I think that we are wandering off the subject. What is important to conclude here is that Bretonia is weak. Well, we already knew that, but why don't the Gauls just storm in then?
Possibility 1: Bretonia is not weak enough to be subdued by a fleet small enough to jump through a jump hole.
Possibility 2: But hey, didn't the Gauls come out of a jump hole in Orkney when they started their invasion? Maybe they actually don't know the locations of the jump holes leading to Bretonian home systems, save the one from Leeds to Manchester.
Possibility 3: The Gauls are waging a war of attrition, besieging Bretonia and expecting the House's surrender. More economical for Gallic troops and hardware.
Possibility 4: Bretonia is, in fact, not the goal of the Gallic conquest. It was merely blocking the path to Liberty. The Gauls don't consider it strong enough to perform significant flank attacks, and it is that very weakness that keeps Bretonia alive.