during the first Boer War, the English decided that war wasn't worth it. A decade or so later, they decided it was. What changed? Gold.
Take away the diamonds, you diminish Cecil Rhodes to a sickly nobody (diamonds made him his money, which he then used to buy influence, which he then used to be the big player in South Africa, Zambia and Zimbabwe, and who dreamed of the cape to cairo railway). He's the founding father of SA, and Z, and Z. Keep the resources, but send Rhodes elsewhere for his youth health issues and you radically change history in that part of the world. Getting rid of the resources, keeps Rhodes, and also radically changes history in that part of the world, with ripples affecting many corners of the globe.
Most likely, the British are happy to control the coast, and let the Boers farm in peace.
Would Bush the elder have defended Kuwait if all they produced was brocoli? Nope. Would England have spent so much coin to dominate southern Africa if all it had was farm crops? Nope.
And, the Portuguese were lucky to manage a wet dream, let alone an African empire, so count them out of the equation.