50mm screed is plenty thick enough for wet UFH.
As for the bathroom, Electric heating mats work great, tiles can be problematic tho. I have 800mm porcelain tiles and they can curl up a bit if/when the cat decides to cook itself all day/night in one spot. several times i have stubbed my toe on a tile edge or corner only for it to be flush again the next day. In future i would go with smaller tiles.