A cursory Google Image Search turned up this one in South Africa, but nothing in Great Britain
Kimberley - Museums and Historic Places
Shouldn't there be?
Absolutely, in my view.
How about Winston Churchill?
There appear to be at least a couple in Great Britain, and his stellar legacy is also honored by The Churchill Centre
Sir Winston Churchill - The Churchill Centre
I agree however with the obvious intent of your question.....Mandela's connection to Great Britain is only an ideological one shared by Britain's far Left. There are other people far more deserving of a statue, particularly in such an important and historic place as Trafalgar Square.