My dad IS sparky, i did this job myself but i didn’t turn the power back on until my dad signed it off (and i’ve worked with him on and off for 25 Years). DO NOT use your earth as a neutral. Fire, fire burning, fire burning and oh.. more fire!!
if you want to run a neutral, make sure your system is wired for a ring circuit on the lights, and then run a new neutral from your ceiling rose (or better yet, replace the entire run to the ceiling rose with a fresh run of 3-core 1 mm flex).
or even better, and by the safest – get a sparky in to do it for you. If the lighting wiring is not set to have a neutral wire, then it’s not a like-for like swap and you shouldn’t be doing this unless you’re qualified AND have part P for domestic installations.
please play it safe and do this properly