AHC/PC: Zulu Empire repels the British

What would be the most plausible POD, following the death of Shaka kaSenzangakhona, for the Zulu Empire to develop the necessary means to repel the British and remain a native African nation that lasts to present day?
 
I'm afraid that such a PoD would have to be very early in order for the Zulu to have a chance of resisting the british and actually surviving. While they may initially repel them, actively retaining total independence is very difficult for the Zulu.
 
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