Like all places, you want to know where you are going and don't go to the dangerous spots (which can also be majority white) in the night time
For the tourist spots it is very safe
I would recommend a nice walk from Victoria Terminus to Buck house to the houses of parliament, then across the bridge to the London war Museum, then back across tower bridge to the tower, then St pauls, The City, then to Trafalgar square and the museums there.
We’re an older group & walking is at least a little difficult for one or two of us. But your walk sounds nice. I’ve been looking at a hop-on, hop-off river cruise that starts at the Westminster bridge & goes down to Greenwich. We’ll be in London for three full days. I’ll add your suggestion to my notes. Thanks! :-)