I'm not familiar with the Brits so can't comment on the specifics there.
However, as a kid, I had a similar experience in a completely different country when we moved cities. My accent wasn't "posh" or "higher class" in any way, it was just from a different region. Kids would give me a hard time for it. But the exact same would happen in reverse form in the other region.
Guess people just don't like "outsiders".