I assume what you meant by that is that only a British-born citizen can be elected prime minister. I am not aware of any such provision in the British constitution.
Britain has a parliamentary democracy -- the original one, actually, what's called the "Westminster model". Brits do not cast a ballot for prime minister. They vote for their local MP, and Parliament elects the PM. In practice, it's the leader of the majority party or the senior party in the governing coalition.
There are foreign-born MPs, and as far as I know they would be eligible to be prime minister if they rose through the ranks to lead one party or another.
Seems to be if he will not pursuit his sport career all the time he can become a prominent politician (like many political family heirs), either in Britain, the US or even South Africa.
A IronMan is maybe something all these countries would need soon, although the first two have had one (his grandmother and Reagan) so maybe it is only fair that South Africa gets him. If the situation in that country will allow it. The new head of the ANC seems to be one to lead it to ruins so maybe someone like him will come along later to fix it.
One question about the born in the US rule. If he is born as an american citizen with american parents, can´t he still not become president because he was not born on US soil? What is then considered US soil? How about all the protectorates in the Pacific, or a military base overseas or something like that?
Merry Christmas all!!