PLEASE DO NOT USE THE EARTH AS A NEUTRAL. If you only have a twin & earth going to your light switch, this is called a switch drop. Perm live and switch live. All uk cables need to have a cpc/earth, it is not designed for the use of carrying constant electricity, only short circuit faults for very short periods. Changing it to a neutral would take away the protective earth from the cable. It is also not insulated and will start to melt the grey sheath of the cable. Fire risk, well give it time it WILL become a fire. Get and electrician in and do it the correct way. We follow British Standards for a reason.