How you react to this kind of thing is subjective though, this particular cop probably felt like he was unfairly having insults regarding his race thrown at him, the news says he was a younger officer as well (I don't believe that's speculation). We do live in an age where it's just easy to offend people now, so just avoiding any references to race/ethnicity/religion when you insult someone seems like it should be common sense. I do agree it's a bit petty from the cop but it still qualifies as a racial slur as per the law. If the roles were reversed the cop in question would be suspended immediately and there would be knives out for the cop - laws should technically protect everyone, not just minorities. She'd probably have no problem if she just called him a wanker/cnut.
Being of a race that would be considered a minority in the UK, I wouldn't be too fussed if someone on a London street threw a racial slur at me, it's just words at the end of the day, and I'd probably insult them back till they got back into their little shell (Probably not worth it, but I don't like to back down from a fight) - but that's just me, someone else could have their anxieties exacerbated, underlying mental health conditions further affected etc.
What's interesting is how the media seems to be reacting to it, there's a lot of stories out there saying it's not racist etc. - I think it's a bit racist personally, maybe not as damaging as a racial slur against a disadvantaged person from a minority. If I had a kid and they berated a white person for being white, being a coloniser etc. I'd tell them off. Minorities should not take this growing understanding around institutional racism, conversations around the impacts of slavery/colonialism for granted imo and use that as an excuse to then throw insults referring to race/ethnicity or religion at the other side - in most cases, the best way to overcome prejudices against you and to change those perceptions is by being the bigger person.