@Ponoka7 I've heard people say stfu as an expression of surprise too. I think this is where tone of voice probably matters lots of phrases can sound aggressive or not, depending upon how they are spoken.
"It seems surprising to me that there are that many people who don’t really interact with speakers of Multicultural London English or Multicultural Urban British English,"
@Takoneko Lots of people in the UK don't live in cities though, so the slang we hear regularly is different.