The comparison is not as apt - Rome and Persia bordered each other - England is on an island, and separate from the mainland. Orlais doesn't have to build ships to invade - not, of course, that it stops either France or England in other wars.

Well, and the fact that Word of God says that's the parallel they were going for, which is why I used it. They've described the Orlais/Ferelden relationship as the inverse of the French/English relationship, with France as the occupying power.

And I'm not arguing that it wasn't considered. But I don't think it was considered for very long. If you accept that Celene is not an idiot, again, there's nothing she could get from a military occupation that she couldn't get more cheaply by some other means--like, say, marrying Cailan.