I was just in England (when she won that race). The locals were amped! It makes sense. We identify with people we think we are "similar to" and even though 0% of actual English people are perfect human specimens like Keely, they still like to think that they can see themselves in her. They went to the same kind of school she went to, have parents who like like her parents, and ran in the same kind of middle school meets she ran in.
The same way a random D3 kid identifies more with Cooper Teare or Cole Hocker than they do with Lagat or Khalid Khannouchi (both Americans).
So while you can call it racism, it just has more to do with a sense of "self" that maps onto the fact that Keely feelsl ike someone they might have known from their hometown. After all, she grew up near Atherton/Manchester, which feels very "normal" to most English people.