It's hard to talk through the comparison since it would be difficult to find people would have lived in both countries enough to know if it's true, but I wonder if K$ in the UK is kinda like what Jessie J is in America. We're aware of her and we know some of her songs, but she's not really a celebrity or star at all here.
Out of curiosity since you mentioned Katy (this is only briefly off topic mods

), how big are the rest of the American stars there (Brit, Beyonce, Gaga, Katy, Rih)? How are they perceived?