South African capital ships, Simon's Town, and imperial sea routes

Could South Africa realistically operate an older dreadnought, and why would the Royal Navy place one there?

Yes, if Royal Navy training and support built the local service gradually, while Simon's Town and the Cape route gave the deployment strategic value beyond colonial policing.

Simon's Town mattered because the Cape route offered Britain's alternative connection to India and the Far East when the Suez route was unavailable. Self-governing Dominions also occupied a different place from colonies in imperial naval planning. A proposed South African-owned capital ship could begin with Royal Navy training and supporting infrastructure while local capacity developed. It would initially be a de facto Royal Navy unit, with Dominion personnel able to move through the wider service. Overseas basing could spread the infrastructure and crewing burden while leaving the ship available for recall.