Probably their approximation of received pronunciation.

Americans have really shitty ears. Any white person who sounds anywhere between "Chimney Sweep in Mary Poppins" and "King George in Hamilton" is automatically classified as British.

And yes, I know both of those characters are originally portrayed by American performers.

Black, Indian, and Asian people with a British accent make our brains explode.

Well we can probably deduce that it wasn't welsh due to the lack of consonants.

Edit: realised seconds after posting that welsh is probably considered a language.