The only
Manfred I've ever heard of in South
Africa is Mr Mann! And I was a teacher here for a lo-o-ong time.
Having said that, Afrikaans-speaking South Africans do tend to use
German names, though not as much as one might expect.
German was the default foreign language on offer in Afrikaans schools, and I wouldn't be surprised if there weren't a few Manfreds dotted about somewhere. But as I say, I've never encountered one, English or Afrikaans speaking.
In England, I would guess that obviously
German names would tend to have been rejected from around 1914 onwards. Just as one World War faded a bit from memory, along came another one. But it is a guess. And it doesn't always work, as there can be other factors such as using saints' names in RC families.
Ursula is an example there. Was there a St
Manfred, I wonder?