I just read Mark Mathabane's Kiffir Boy for class and have been researching the topic.

While it would seem that British imperialism in South Africa seems to have created the strongest economy in Africa since its independence in 1961, it was the apartheid regime that caused most of this economic growth. The white population of south Africa had the highest standard of living in the world around 1980 and this was due to their ability to exploit the native population until the end of the 20th century. Among the most sinister policies that the government enacted was the passbook system which was an internal passport system that forced the South Africans to live in their 'homelands,' which were state appointed arid lands that couldn't support half the population meant to live there, unless they had proof of a job in a city. If someone had a job and lost it (which could be for any reason) they had 72 hours to find a new job before being an illegal alien in their own country. There were no jobs on the homelands they were supposed to move back to so there were huge illegal populations in cities that were so desperate for work that they would accept almost any job. This created the labor pool that the Afrikaners needed to create their advanced economy.

As far as British development, one of the biggest ways that they helped the Afrikaner economy was turning a blind eye to the exploitation in the Apartheid Regime, along with US. While the US would have been hypocritical at the time, both the US and UK ignored the human rights problems because South Africa was a capitalist wall against Soviet imperialism. It is not a coincidence that the dates of the Cold War and the Apartheid dates are very similar (Apartheid 1948-1994). While I'm sure Britain did leave vital infrastructure in place, it was the Afrikaner population, which Britain had gone to war with and sought to contain, that really created the economic growth.

Sources/Recommended Media:

Last Grave at Dimbaza (1974) - Documentary which was filmed illegal within South Africa showing the struggles in the homelands and shantytowns. Really moving documentary that helped instigate change.

Twentieth-Century South Africa by William Beinart - Good base text for the time period. Goes into the source of the economic growth I described

The Kiffir Boy - Autobiography of Mark Mathabane who describes his childhood struggles in a South African illegal slum. He later moves to the US on a scholarship for both academics and tennis. Really interesting first person perspective for the time but a bit off the original question.