Gonna be refurbing my bathroom and i'm going to install UFH. I intend to cover the heating cable with Mapei Fiberplan. Just wanted to know if the whole floor needs to be covered as i was intending to leave the area beneath the bath and shower ?

Just the area you will be tiling. No ufh under the bath is there? I hope not .
You will have to place something on the floor to stop the leveller running to areas you dont need ie, under the bath.

best would be to baton round the room 1st with wooden batons i.e door reveal/round bath area and any other skirts etc which are missing so the slc doesnt go everywhereseal edges of batons with silicon

as above just slc tiled-to-be areas, damm perimeters with battens

i would agree as long as you do not intend to tile these areas ! if you do intend to tile under the bath then it would be best to self level as you will use less adhesive when tiling

I'd SLC the entire room, as it adds to the stiffness of the substrate, and for a bunch of other reasons as well. If you want to partition the SLC, you could use something similar to that in the attached picture. It's a foam tape barrier which you can use to avoid getting the slc where you don't want it.

Thanks for all the advice. I'll shutter the SLC off with offcuts of ply and silicone the joint.