You will need to lift the floor above the ceiling, trace a LNE from the lighting circuit junction box or rose and extend that on to the new light position. Then you need to run a cable between the light and the switch location.
It might be better to look at the other lights in area, say the hall light to see if any of the lights have the LNE at the fitting, if none do you may find there's a central junction under the floor serving all lights for the grd.
If you keep to plastic switches, plastic faceplate screws and simple pendant lighting the earthing isn't critical.
Some will argue it is life and death, but I'd suspect they have forgotten that maybe 40% of all housing stock doesn't have earths on the lighting circuits.
Same with the RCD, It is a regulatoryl requirement on new installations, and add on to circuits, but can be avoided if you keep the cable over 50mm from the surface. That isn't a hard task in a floor void, and might not be a problem if the switch cable drops in to a wall void.
I'll put the flame proof suit on, because it's very likely that I'll be roasted by the righteous who would condone my suggests as being not to current spec.
They would be right, but then again as mentioned maybe 70% of all houses must be due a fire, death by electrocution all because they aren't to 17th ed current regs.