Possibly in recent years this has become true.
But I was stationed in England for three years and visited their several times afterwards. In my experience, this was definitely not the case. Speaking to a black British person sounds the same as speaking to a white British person, and they are equally polite. No be talkin' ghetto jive. But like I said, maybe not anymore.
Some British blacks are still like that. Some of the younger ones have sadly been corrupted by the influence of African America’s dumbass ghetto culture and adopted America’s racial grievances as their own, despite the very different history and circumstances that led to black people living in Britain.