The booing made it quite clear how unhappy with that development many within the ANC still were. So much for a Communist dictatorship. You cannot do that in one of those.
Yet even Ian Smith said that he had been, "right about Mugabe, but wrong about Mandela." He ended his days in the new South Africa rather than in the entire country that Thatcher left to the world, which is not Britain, but Mugabe's Zimbabwe.
Norman Tebbit, Terry Dicks, and thus also David Cameron in that he permits them to retain party membership, are therefore more hardline, more right-wing, or however you want to put it, than even Ian Smith was by the time that he died.
Never mind shaking his hand, Cameron ought to have kowtowed to Mugabe. His heroine would have done, and she pretty much did. But Mandela never did, and he never would have done.