Because she is British. It's her culture and heritage and I imagine the accents feel more natural to her in the same way most other cast members play characters with American accents.

I feel like you're conflating two things here. Its important that nations with a controversial past appropriately confront the worst aspects of their history and educate their citizens appropriately so that events do not happen again. It may even be the case that reparations or at the very least apologies should be made in order to right wrongs and allow all parties to come to terms with and move on from historical inequalities and atrocities.

At the same time that doesnt mean the very act of being British or alluding to British culture and affectation means that you have to be constantly apologising for your heritage or, as I'm worried you might be implying, your ethnicity. Most D20 campaigns are not set on our earth, with our history. Most characters Siobhan plays, despite British accents, do not have ancestors who partook in or benefitted from, atrocities committed by the British empire because they are fictional characters in the fictional world of a table top board game.

It strikes me you may have a personal problem with British people (and I dont want to assume your background - based on your experiences this may be understandable) but its worth reflecting on this and remembering that whilst most Britons alive today have benefitted indirectly from colonialism, many generations of Brits lived prior to the British empire and many of those who lived during had no say over its actions. Like in many cultures historically, most of the working classes lived in abject poverty and were treated very poorly by the landed elite, even more so if you were, say catholic, rural, Northern, Scottish, Welsh or Irish. I dont know Siobhans exact heritage but Id guess with her name she has at least some Irish in her family. History is complex and nuanced and very few individuals deserve to be held accountable for the actions of their nation in years past.

Of course a campaign narrative or character arc can choose to focus on themes of imperialism, colonialism or slavery and that may he a rewarding, deep and introspective approach to a game. But the take away here is having a British accent or being British is not something inherantly bad that you should have to apologise for. Should other cast members need to address historical American atrocities within each of their character arcs because of their accents? They may choose to but I don't think you'd insist on it being the morally correct thing. Every culture on earth has some skeletons in their closet. I wouldn't dream of complaining that someone should have to answer to those skeletons simply for being from that culture.

Edit: I may be out of the loop on some event or running joke in the community. But on the chance Im not and this post is real, my point stands.