Adding a light to my porch?

Typically a lighting cable carrying Live Neutral & Earth is taken to the place where the light will be (ceiling rose) and this will often come from another ceiling rose and must be part of the lighting circuit.

Another cable from the ceiling point runs off to the switch position carrying permanent Live out to it and Switched Live back to it - to turn the light on and off.

Light switches are usually designated Com L1, L2 - the usual connection protocol is to place Permanent Live in Com (common) and Switched Live in either L1 or L2 it really doesn't matter which. With two each of Com, L1,L2 you appear to have two switches on the same plate - a Two-Gang switch for two separate lights.

You do not mention the provision of lighting power in your porch - just the cable run from ceiling to switch.

An electrician may be your best bet.

Good luck...

Peccavi
December 2009