I'm aware of Rwandair to Kigali via Brussels. As we're all aware of, Rwandair won't be returning to LGW although I believe that it's the airline's intentions to eventually fly non stop to LHR from Kigali.
Uganda Airlines was expected to start Entebbe to LHR on the 28 March 2021 although it hasn't started yet.
Another airline that prior to the pandemic had announced it's intendion on returning to London is Air Tanzania. I'm sure we're be reading more about their intentions within the next few months and I should imagine that they will also be in a position to obtain slots at LHR for their Dar Es Salaam flights.