I’ve seen it suggested (and possibly confirmed in some book or other, I can’t remember) that in-universe, the English accent = Core Worlds, American = Mid/Outer Rim. I like this explanation a lot because the implication is that the Empire’s officer corps is disproportionately drawn from Coruscanti recruits while most of the stormtroopers are from the sticks, but it’s kind of blown to hell by the fact that Corellia is always depicted as being in or near the Core and Han definitely doesn’t have an English accent.