I'm Brazilian and I have a Blue Card from working in Germany. I want to visit London. Do I need a visa to do that? I know that Brazilians don't need a visa to visit but, since I'm not travelling there from Brazil, I'm not sure.
It doesn't matter where you're flying from. Either you need a visa for the UK or you don't.
Like you said, you can enter without a visa, and can usually stay for up to 6 months.