South Africa House - the South African Embassy - is on the east side of Trafalgar Square, and was therefore the physical manifestation in this country of the evil regime of apartheid... just as, today, it is the physical manifestation of the dream of a rainbow nation that Mandela has done so much to make possible. It was also the site of lengthy demonstrations against apartheid and against Mandela's imprisonment.
I'm really not trying to tell you what to think, but I can't believe that anyone who thinks about this would have "no interest whatsoever in him."
As for the statue: Isn't it the sign of a mature, confident society that we can consider celebrating the life and contribution of a good man such as this even though he isn't from these shores?
In a quest for Englishness, I suggest you also change the name of the square. Cape Trafalgar is in southern Spain.
(Incidentally, I think you'll find there's already one or two statues of famous Englishmen in Trafalgar Square - Lord Nelson, Charles I, George IV, Havelock and Napier. I suggest there are several Englishmen even more famous than Havelock and Napier - David Beckham, for example, or Chris Bisson.