It's obviously not as...ahem...a black or white an issue as that.
Minorities in this country (and others, obviously) have historically been discriminated against, oppressed, abused, ignored and even killed due to nothing more than the colour of their skin. This still happens today, everywhere from police stops, to housing, education, employment, medical diagnosis, finance, even simple day-to-day interactions...this is not up for debate.
Things are far from perfect, but they're better than they were as society takes actions to redress the balance.
A minority celebrating their culture or their community isn't simply a case of celebrating the colour of their skin, or their heritage. It's about celebrating the progress made. That a black person can be a high ranking police officer, or a corporate CEO, or a bank manager, where for previous generations that was not an option. It's about redress, or being treated equally. Being accepted, while remembering your roots. Being black, or brown or whatever on an equal footing in a society that has historically been preferential to white people and considered you lesser. Breaking stereotypes.
Can people not see why a minority might be proud of that progress, and feel the need to celebrate it.
Being proud of being white is...what? White isn't a culture. White people have never been oppressed in this country just for being white. Don't conflate it with being proud to be English or British or whatever. The only thing unique about being white in England is that you've (knowingly or otherwise) enjoyed the benefits of a society that has been kinder to you and easier for you by virtue of a genetic lottery that granted you a certain skin pigmentation. A white person having a tough life doesn't negate the experience of minority groups.
Celebrating 'being white' in England, a country where people of all colours are as culturally English as you, is to suggest that you are somehow better, more important or more entitled than someone who isn't, by virtue of the colour of your skin. That what the Nazis believed. That's what the KKK believes.
Thankfully it isn't true. A white person in England who's biggest case for celebration is being white must have absolutely nothing else going on in their life.