Give a sellpoint to Southampton shipyard too, then. Since the Bretonia carrier is going to be mass produced in the future, along with other already produced bretonian warships, you'll have a high demand for Iridium (for industrial materials and reactors) anyway.
Holstein as a low(er) risk, lower profit (comparably shorter route -> less piracy opportunities).
Valetta for a lot of unlawfuls -> cargo piracy
Southampton for high risk high reward (longer route -> more piracy opportunities).
