We had a similar question about a London bank being used for a "C/o" address.
My suggestion (I don't know this for a fact) was that people emigrating from South Africa to Britain might need a Poste Restante address, till they got settled, and used the british branch of a local S.A. institution for this.
In the previous question, the London address was for a branch of a S.A. bank. In your example, the South Africa House in London is being used.
Possibily you need to be in Britain already, before you can arrange a P.O. box as an address.
Bob
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Searching for Coleman, Moore, Kallnung in London; Margulies, Remenyi in E. Europe; Ancestors of Hessie Stevenson-Coleman-Baxter (Ireland, 1861) and, of course, any other ancestors for my web-site.
I do know that the family did go back to South Africa.
My husband does remember that a member of the family worked or was commissioner of trade for South Africa? but he was only young at the time and can't remember details.
I will have to do some more digging to see if I can find a connection.