I'm no electrician, so am not qualified to advise, but...
Remove power. Make sure that you've really removed power.
You'll find a switched live and a neutral in the ceiling rose in the bathroom, plus earth. Take the switched live, run that to one side of a switch in the loft or wherever you want the switch for the loft to be. The other side of the new switch goes to one terminal on the light in the loft.
The other side of the light goes to the neutral in the ceiling rose.
Don't blame me if it goes bang.
