I think a - brits to the north, NNC to the east - the NNC are the weak link , so the trench in the east will help.
Artillery - again a. We need the firepower to help break the main attack from the north. By spreading the guns, the infantry will help support the guns when the Zulus get close. The rockets are more of a morale breaker, so again, near the main attack.
Cavalry - I think b - we need a mobile reserve to shore up where we are pressed most - perhaps a small number say 5 could be dispatched for help.

Finally, we need to get as much ammo as possible up near the firing lines of British troops - there will be plenty of targets soon!!