I think Mercian Billman has it right - just enough forces to keep the Zulus running - and the cav can disengage if new threats occur - and let us help the forces to the north as soon as possible.
The intervention of the cavalry was an interesting one - Zulus were particuarly unsuited to facing them - no missle weapons to talk of and no long spears. It is, of course, no surprise that they weren't prepared as none of their foes, until the British army, rode horses to war. The Boers used horses, but just as a method of getting to battle, not fighting in it. In subsequent battles in the Zulu war the charge of the cavalry was the climax of the battle and the straw that broke the Zulu's back.
I feel we are doing better than real life, but it could still turn again.