Nearly 7,500 people have registered to vote in London.
But BBC Africa analyst Martin Plaut says this is only a small percentage of the community in Britain, which numbers hundreds of thousands.
Those who had come were excited to be there.
"I am proud to participate in a democracy we worked so hard to achieve," said one voter patiently waiting at South Africa House, where anti-apartheid activists once demonstrated for Nelson Mandela to be released from prison.
South Africans living abroad were only given the vote after a court ruling was confirmed in March.
The opposition Afrikaner nationalist Freedom Front Plus party had brought the legal application on behalf of a South African school teacher living in the UK.
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