I'm not ignorant. Prejudice is always wrong. It's just this thread isn't about racial prejudices towards travellers.
Then maybe don't make blanket statements that simply aren't true.
If you state that white people cannot suffer from racism then I think you have to accept that people will challenge you. If you don't want to be corrected why say it?
If you post about the racism that black people face I very much doubt anyone would argue with you, and if they did I would be challenging them.
But if you post an incorrect or inaccurate statement then I think you should be challenged.
I'm not even bothered about you posting about white British people, but there are specific communities of people, who happen to be white who do suffer considerable racism and persecution - the Travelling community, who are unwelcome practically everywhere, are repeatedly evicted and moved on, and the Jewish community who suffer ant semitism even within political parties. Orthodox Jews are easily identifiable too so you can't claim that their white skin disguises their identity.
You want white people to examine their prejudices and I absolutely agree with you. We all should. Sadly I think the worst offenders are least likely to do so but I think actually that you should examine your prejudice against people of other ethnicities.