it wouldnt even be a slither it would look bad imo, boxing out would look crap too due to the sill being in the way and it not being a straight run
So packing 20mm of adhesive behind the tiles would look better? That's a chunky bit of trim needed.
I am right in thinking it's only around the bath being tiled?
Last thing I would be doing is packing out a whole wall for the sake of the bath being 1" short.
Two options the way I see it.
1. Batten and tile a small ledge
2. Let the tile fall below the bath and depending on the gap left, silicone seal.