I am from Mali, and I have a Refugee Travel Document, a TVR as we say in French, otherwise called a "refugee passport." Do I need a visa to go to London?
You will need a UK visa to enter the UK.
This UK government page says:
Holders of non-national and refugee travel documents
If the passenger holds a refugee travel document issued by the UK they do not need a visa. If the passenger holds any other non-national or refugee travel document they need a visa to enter the UK. (emphasis added)