We are both assuming that you can get access to the end of the cable that feeds this light. If it has conventional grey cable it will be twin core plus earth. We understand that the outer sheath has been cut back somewhat and the earth sliced away. But if you can access the cable itself the cheapest and safest way to reconnect it would be to mount a little circular 10A junction box somewhere close to the end of the old cable, connect up a short length of new cable to bridge the gap to the ceiling fitting.
Looking at the wording of your post again, you refer to the light switch. The light switch, if it mounted half-way down the wall has no bearing on the earthing system for the light at all. Look at the image on this web link (down the page). Does that help explain where the earth connection goes? - and that it is not connected through the switch
http://www.diydoctor.org.uk/projects/lights_an d_switches.htm