For the sake of comparison, at the beginning of the Gallo-Bretonian war, Gallia had 500 battleships, while Bretonia had 30, although "only" 150 of the Gallic ones were performing the invasion. This is Igiss' lore. The remaining 350 were left to keep the home safe, which is not a terribly simple task if you're an autocrat.
Bretonia hasn't actually fought in any, at all, major battles against the GRN. At least by the lore, in game stuff is just for fun. They employed a strategy similar to the Russians against Napoleon. Retreat, scorch behind you. Leave the stations, but take every useful system which you can from them. Tell some Kossaks to fire a few shots when they see fit and then to run. The only notable engagement was the battle between the Villeneuve and the Derby in the Gallic invasion of Leeds, when the Derby rammed the Gallic command ship in order to give time for the defenders of Leeds to retreat and, as tradition goes in Disco, blast the jump gates.
Leeds was a major industrial center, that is true. Without it, Bretonian industry is weaker, that is a fact. A considerable part of what could be moved from Leeds elsewhere, however, has been moved, so we can expect acidic rain on New London soon, since that is where the main portion was moved to. Some of it is being moved to Sprague, but as the planet has been colonized only recently and hastily, it is enough for supporting the southern border against pirates, but hardly enough to help the front. The loss of LD-14 as an MOX (fuel) producer was important, but most of its elements have been moved to Birmingham Station in Manchester, which is compensating its lost function to some extent, which you can notice in-game by the higher fuel cost (it costs the highest in Bretonia out of all the Hosues due to LD-14).
Meanwhile, trade with Liberty and Rheinland is bustling, while goods exchange with Kusari (through other houses ofc) is rising since the war's end.
What Bretonia is relying on is that all the jump gates leading to its core territory from Gallic controlled space have been disabled. There are some jump holes, yes, but those always end in asteroid or waste fields and are unstable. The Gauls can use them to send small raiding parties, such as the stationary Valor in Manchester is (oops, forgot the name), but sending large fleets through there would be a huge gamble where the Bretonians would have an advantage.
The only hole in such defenses is Magellan, since the jump hole to Leeds is almost in clear space and apparently stable enough for Liberty to launch Operation Royal Flush. When (if?) the Gauls capture Magellan, it will be their choice whether to go to Manchester or to California -- or both.
tl;dr: apparently Bretonia relies on no reliable connection existing between their space and the one controlled by the GRN.