Non-British British Person

Huh? Did you grow up in a southern area? Children develop accents based on most of the language they hear around them, not just from the parents. That's one reason why a child of parents with speech impediments can talk normally.

I grew up in Arkansas, and when I was growing up it turns out my parents had my relatives mask their accents as well, so I grew up without a southern accent, and I was home-schooled up until fourth grade, so by then I didn't have a southern accent.

Okay, so it's possible that severely limiting social contact and having everyone around the child fake an accent could produce another accent, but come on. Occam's razor, folks. It's way more likely that the kid from camp in the story faked a British accent while at camp and then lied about it.