This actually happened to me but I was totally dismissive of it at the time. I was hit by a car once and woke up from the surgery with what I can only describe as a cowboy accent. I had no clue what to make of it but within a month or two it gradually wore off and I just completely forgot about it until now. Super weird.
The accent is probably from her subconscious. It could have been triggered by something that happened to her and (her voice) became British because somewhere in her life knowingly or unknowingly she heard the British voice and it got stuck in her head.