This MSN article from 2018-02-13 claims that an American woman fell asleep with a bad headache, and woke up with a British accent.
This has been widely reported, including by The Washington Post and The Independent. There have been other reported cases throughout the years, such as this ABC News article from 2011 about another American woman who also developed a British accent following surgery.
The articles all attribute this to a very rare condition called Foreign Accent Syndrome - the articles state that there have only been around 60 known cases, and it is often preceded by a stroke.
The 2011 ABC article that I linked to above has a quote from Dr. Ted Lowenkopf, a neurologist and medical director of Providence Stroke Center in Portland, Oregon, which makes the condition a lot more plausible:
Although Butler's accent sounds vaguely British -- Welsh, even -- it's purely coincidental.
"Although we think it sounds like a British accent, if you had a language expert listening to her, they would say that's not an English accent," Lowenkopf told KATU. "It's sort of an amalgam of different-sounding speech that sounds like a foreign accent. But it's not truly typical of any one foreign accent."
I know that brain injuries affect language all the time (e.g. Broca's aphasia), but is Foreign Accent Syndrome a widely accepted condition in the medical community? Does it always result in a "British" accent (similar to how a lisp will always result in difficulty with the same phonemes)? Has this been documented in non-English speakers?