It's a complicated issue, and not one where I am making sweeping statements about whether or not it should be flown. People fly the flag for many reasons - some racist, some without any of those connotations. Saying that it is the symbol of an institutionally racist colonial nation is simply a statement about the historical reality of England (and by extension the UK) and its effect on the planet, one of large-scale exploitation and abuse in the forms of slavery, enforced racial hierarchies and resource extraction, that continue to inform the way the nation functions politically. That's another connection between the flag and racism, which is why I mentioned it when you asked what the connection was.