In OTL, the Boer kommandos got bogged down when they attempted to besiege the main British force in Natal in the town of Ladysmith. Although both sides suffered in the siege, the Boers were hurt more as with no real action many of their burghers drifted away- at this point in the war the Boers had had numerical superiority but with the time wasted at Ladysmith, the British were able to land more troops at Durban. The initial Boer offensive had been a great success with the kommandos driving the British back but with the siege of Ladysmith, the offensive totally lost momentum.
What happens if the kommandos leave the British in Ladysmith and continue with their original plan of a swift blitz to Durban? The British garrison would be in their rear, true but it would be unable to actually take the offensive- with the superior mobility of the kommandos, flying columns could easily be despatched to harry the British if necessary. If Natal is overrun can the British government be convinced to seek terms?