That's not true, we regularly judge people not doing things the 'right' way, but we just don't use the term 'un-British'. It's hard to explain, but there are code-phrases that some use like 'its not the done thing', or 'they're not our sort of people'.
Supporting football's not really 'British', and a fairly modern phenomenon. In the 80s it was deemed uncouth and heavily associated with hooliganism, but rehabilitated in the 90s, and is already falling out of fashion again.
Football support is heavily wrapped up in the complicated British classes, in the 90s/00s it was cool to pretend to be working class when you were middle class, and supporting football was a visible way of doing that.
Supporting football's not really 'British', and a fairly modern phenomenon. In the 80s it was deemed uncouth and heavily associated with hooliganism, but rehabilitated in the 90s, and is already falling out of fashion again.
Football support is heavily wrapped up in the complicated British classes, in the 90s/00s it was cool to pretend to be working class when you were middle class, and supporting football was a visible way of doing that.