The posts by Peter (from SA) in the old thread, #34 and #37, are likely the best explanation available for the absence of both British and SA property stamps of wartime vintage.

Why there are also no post-war commercial proofs can only mean they were not offered for sale as surplus in Britain, for reasons that remain unexplained.

That in itself is not so unusual either, though; we do encounter standard BSR‘s without such proofs here not infrequently. Many served as police or military arms in smaller newly independent nations of the Commonwealth after WW II, and were later gathered up and brought back to the US surplus market by the likes of Sam Cummings without ever seeing Britain again. Before 1968 there would be no import markings.