In the trooper storyline, the imperial accent is reffered to as pure Drommund Kass
In general, people associated a Vaguely British Sounding Accent (RP and all its many derivatives and variants) with the Galactic Empire because of Tarkin. The difference went down for many fans' headcanon as a Coruscant (eventually just "Core Worlds") accent, with American English sounding like a Corellian accent. It wasn't a political divide - Obi-Wan Kenobi's accent was clearly British, as were those of other Jedi that showed up in the prequels - but a cultural one. This would obviously be an oversimplification, but it gained significant credence as later Star Wars
EU authors played up the cleavages between Core culture, Rim culture, and the whole Corellian-dominated section of the galaxy. Language and accent is commonly an aspect of culture, so to go along with these different cultures we had different accents. So.
SWTOR basically turned that on its head. The Sith Empire obviously has nothing to do with Palpatine's Empire; Palpatine's Empire was a corruption of the Republic, and it was three and a half millennia distant. It also has nothing to do with Core/Rim cultural divides, especially since Dromund Kaas and the rest of Sith Space are out in this tiny corner of the Outer Rim itself and don't even control
any parts of the Core. The reason Sith and Imps speak in their various kinds of British accent is not for any reason connected to lore, therefore. It's more mundane. The devs wanted to make the Sith Empire seem
like Palpatine's Empire to keep everything in terms of a Star Wars
that even casual fans could understand quickly. So they went with tropes that they thought people associated with Palpatine's Empire for the Sith Empire: the design of their spaceships, the cut of their military uniforms, even the emblem.
This wouldn't really have been a problem except some of the actual quests took that even further. The description of Elara Dorne's accent as "pure Dromund Kaas" looks bizarre to anybody with even a passing familiarity with Star Wars
lore - it's by and large the same accent that Bastila Shan, Obi-Wan Kenobi, and Ki-Adi-Mundi share. Now, of course, Sgt. Jorgan could be wrong
in that description, but it's not the only instance of that showing up. The Agent, Warrior, and Inquisitor all change their voices when attempting to pass as "not Imperial" in various contexts (e.g. the Foundry flashpoint, or the Agent's undercover mission on Nal Hutta). Clearly we're meant to think that that accent really is
restricted to the Sith and the Imps. And if that's the case, how could it be the "Core accent" three hundred years before and three thousand years after the game?
Just another one of those little oddities of continuity that can be annoying about SWTOR.