“Just pass it thro the conduit” — easier said than done. Do you have experience of UK wiring? Most household wiring is done in flat PVC sheathed, PVC insulated solid copper cable with an uninsulated circuit protective conductor (cpc/earth/ground wire), known as 6242Y or “twin & earth”. This is often buried in plaster and sometimes capped with a thin, open-backed channel, while even in a hollow stud wall the cable may be clipped and not free to move or be replaced. Dot & dab plasterboard adhered to block is common in new builds, with cables run between the board and block. None of these scenarios involve conduit, and all of them require an electrician. Running a neutral to the switch is becoming a more common practice, but is not a requirement. The simplest way to do so is to employ 6243Y (three-core and earth) using the three insulated cores as L, switched L, and N. So sure, if you’re doing a rewire, have at it. And if you do happen to have conduit, yes, you can run a single from the ceiling rose to the switch without cosmetic damage, assuming there’s room in the conduit and you can pull past any concealed bends. In short… easier said than done.