Another POD is having Southern Rhodesia vote to join the Union in 1922, rather than decide to go it alone.
Most whites in Southern Rhodesia were English-speakers, so they'd definitely lean towards the South African Party, rather than the Nats. Not sure if this will butterfly away the SAP-Nat fusion of the early 1930s, but it may well do so.
This means that the SAP stays in power, perhaps indefinitely. Or a more pro-British party arises again, similar to the earlier Dominion party. This would likely draw support from the Rhodesians and Natalians.
As others have said, the UP winning in 1948 is another possibility.